* 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>
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