This allows us to match URLs inside link hrefs.

The class helps out with keyword mutes, not just some general concept of
"filtering".

Also add HTML entity decoding to Glitch::FilterHelper, which is needed
to e.g. match "<" to the tag-stripped version of "<p><3</p>" or
"<p>&lt;3</p>".

=~ 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.