Creates filter for REWRITTEN_TAGS that converts them to `p` rather than ripping them out entirely, and formats `ul` as break-separated list
Both changes align sanitization to Mastodon's "strict" sanitizer at https://github.com/mastodon/mastodon/blob/main/lib/sanitize_ext/sanitize_config.rb#L73
I don't love the complexity of the Filter, but Bleach doesn't give us great options to work with. The code operates within an iterator without the useful "sibling" methods that Ruby's equivalent has. Also, Bleach runs filters _after_ sanitizing (unlike Ruby's which runs before) so we have to pass all the elements through the sanitizer, then rewrite them after the fact.
Also fixes an apparent bug in `imageify_emojis.replacer` where `include_local` was not being used correctly (previous code path never returned anything.
Don't send a list when populating Accept header. This seems to work fine on Mastodon and friends, and fixes a bug with the Mitra server until resolved upstream.
Andrew Godwin