* Use a tree‐based approach for adv. text formatting

Sanitizing HTML/Markdown means parsing the content into an HTML tree
under‐the‐hood anyway, and it is more accurate to do mention/hashtag
replacement on the text nodes in that tree than it is to try to hack it
in with regexes et cetera.

This undoes the overrides of `#entities` and `#rewrite` on
`AdvancedTextFormatter` but also stops using them, instead keeping
track of the parsed Nokogiri tree itself and using that in the `#to_s`
method.

Internally, this tree uses `<mastodon-entity>` nodes to keep track of
hashtags, links, and mentions. Sanitization is moved to the beginning,
so it should be known that these do not appear in the input.

* Also disallow entities inside of `<code>`

I think this is generally expected behaviour, and people are annoyed
when their code gets turned into links/hashtags/mentions.

* Minor cleanup to AdvancedTextFormatter

* Change AdvancedTextFormatter to rewrite entities in one pass and sanitize at the end

Also, minor refactoring to better match how other formatters are organized.

* Add some tests

Co-authored-by: Claire <claire.github-309c@sitedethib.com>