Also addresses #463.

Also add an apply_to_mentions attribute on Glitch::KeywordMute, which is
used to calculate scope.  Next up: additions to the test suite to
demonstrate how scoping works.

This has a couple of advantages over the regex approach:

- Keywords are individually addressable, which makes it easier to gather
  statistics (#363)
- Keywords can be individually applied to different feeds, e.g. skipping
  mentions (#454)

It *does* end up creating many more Regexp objects.  I'm not yet sure if
the difference is significant.

=~ made sense when we were passing it through to a regex, but we're no
longer doing that: TagMatcher looks at individual tags and returns a
value that *looks* like what you get out of #=~ but really isn't that
meaningful.  Probably a good idea to not subvert convention like this
and instead use a name with guessable intent.

We already know about one regex limitation, which is that they cannot
segment words in e.g. Japanese, Chinese, or Thai.  It may also end up
that regex matching is too slow compared to other methods.

However, the regex is an implementation detail.  We still want the
ability to switch between "occurs anywhere" and "match whole word", and
caching the matcher result is likely to still be important (since the
matcher itself won't change nearly as often as status ingress rate).
Therefore, we ought to be able to change the cache keys to reflect a
change of data structure.

(Old cache keys expire within minutes, so they shouldn't be too big of
an issue.  Old cache keys could also be explicitly removed by an
instance administrator.)

It is reasonable to expect someone to enter #foo to mute hashtag #foo.
However, tags are recorded on statuses without the preceding #.
To adjust for this, we build a separate tag matcher and use
Tag::HASHTAG_RE to extract a hashtag from the hashtag syntax.

When given two regexps, Regexp.union preserves the options set (or not
set) on each regex; this meant that none of the multiline (m),
case-insensitivity (i), or extended syntax (x) options were set.  Our
regexps are written expecting the m, i, and x options were set on all of
them, so we need to make sure that we preserve that behavior.

@regex can no longer be nil, so we don't need to check it.

Also make the keyword-building methods private: they always probably
should have been private, but now I have encoded enough fun and games
into them that it now seems wrong for them to *not* be private.

It is possible to cache a Regexp object, but I'm not sure what happens
if e.g. that object remains in cache across two different Ruby versions.
Caching a string seems to raise fewer questions.

Regexp#=~ returns nil if it does not match.  An empty mute set does not
match any status, so KeywordMute::Matcher#=~ ought to return nil also.

Ditto for ending with \b.

Consider muting the phrase "(hot take)".  I stipulate it is reasonable
to enter this with the default "match whole word" behavior.  Under the
old behavior, this would be encoded as

    \b\(hot\ take\)\b

However, if \b is before the first character in the string and the first
character in the string is not a word character, then the match will
fail.  Ditto for after.  In our example, "(" is not a word character, so
this will not match statuses containing "(hot take)", and that's a very
surprising behavior.

To address this, we only add leading and trailing \b to keywords that
start or end with word characters.

There are two motivations for this:

1. It looks like we're going to add other features that require
   server-side storage (e.g. user notes).

2. Namespacing glitchsoc modifications is a good idea anyway: even if we
   do not end up doing (1), if upstream introduces a keyword-mute feature
   that also uses a "KeywordMute" model, we can avoid some merge
   conflicts this way and work on the more interesting task of
   choosing which implementation to use.

Word-boundary matching only works as intended in English and languages
that use similar word-breaking characters; it doesn't work so well in
(say) Japanese, Chinese, or Thai.  It's unacceptable to have a feature
that doesn't work as intended for some languages.  (Moreso especially
considering that it's likely that the largest contingent on the Mastodon
bit of the fediverse speaks Japanese.)

There are rules specified in Unicode TR29[1] for word-breaking across
all languages supported by Unicode, but the rules deliberately do not
cover all cases.  In fact, TR29 states

    For example, reliable detection of word boundaries in languages such
    as Thai, Lao, Chinese, or Japanese requires the use of dictionary
    lookup, analogous to English hyphenation.

So we aren't going to be able to make word detection work with regexes
within Mastodon (or glitchsoc).  However, for a first pass (even if it's
kind of punting) we can allow the user to choose whether they want word
or substring detection and warn about the limitations of this
implementation in, say, docs.

[1]: https://unicode.org/reports/tr29/
     https://web.archive.org/web/20171001005125/https://unicode.org/reports/tr29/

The intent of the previous concatenation was to minimize object
allocations, which can end up being a slow killer.  However, it turns
out that under MRI 2.4.x, the shove-strings-in-an-array-and-join method
is not only arguably more common but (in this particular case) actually
allocates *fewer* objects than the string concatenation.

Or, at least, that's what I gather by running this:

    words = %w(palmettoes nudged hibernation bullish stockade's tightened Hades
    Dixie's formalize superego's commissaries Zappa's viceroy's apothecaries
    tablespoonful's barons Chennai tollgate ticked expands)

    a = Account.first

    KeywordMute.transaction do
      words.each { |w| KeywordMute.create!(keyword: w, account: a) }

      GC.start

      s1 = GC.stat

      re = String.new.tap do |str|
        scoped = KeywordMute.where(account: a)
        keywords = scoped.select(:id, :keyword)
        count = scoped.count

        keywords.find_each.with_index do |kw, index|
          str << Regexp.escape(kw.keyword.strip)
          str << '|' if index < count - 1
        end
      end

      s2 = GC.stat

      puts s1.inspect, s2.inspect

      raise ActiveRecord::Rollback
    end

vs this:

    words = %w( palmettoes nudged hibernation bullish stockade's tightened Hades Dixie's
    formalize superego's commissaries Zappa's viceroy's apothecaries tablespoonful's
    barons Chennai tollgate ticked expands
    )

    a = Account.first

    KeywordMute.transaction do
      words.each { |w| KeywordMute.create!(keyword: w, account: a) }

      GC.start

      s1 = GC.stat

      re = [].tap do |arr|
        KeywordMute.where(account: a).select(:keyword, :id).find_each do |m|
          arr << Regexp.escape(m.keyword.strip)
        end
      end.join('|')

      s2 = GC.stat

      puts s1.inspect, s2.inspect

      raise ActiveRecord::Rollback
    end

Using rails r, here is a comparison of the total_allocated_objects and
malloc_increase_bytes GC stat data:

                 total_allocated_objects        malloc_increase_bytes
string concat    3200241 -> 3201428 (+1187)     1176 -> 45216 (44040)
array join       3200380 -> 3201299 (+919)      1176 -> 36448 (35272)

It would also have been valid to get rid of the attr_reader, but I like
being able to reach inside KeywordMute::Matcher without resorting to
instance_variable_get tomfoolery.

A matcher object that builds a match from KeywordMute data and runs it
over text is, in my view, one of the easier ways to write examples for
this sort of thing.

Gist of the proposed keyword mute implementation:

Keyword mutes are represented server-side as one keyword per record.
For each account, there exists a keyword regex that is generated as one
big alternation of all keywords.  This regex is cached (in Redis, I
guess) so we can quickly get it when filtering in FeedManager.