Message Editing: Editing in the timeline

[jryans]

This includes the UX to enter edit mode, show the editing composer (#9481).

Sending the edit is tracked separately (#9575), so this could local edit only at first.

• switch to editor upon clicking edit in action bar
• render pills
• add auto-completion for pills
• enter to save, shift enter to newline
• only allow editing events of type=m.room.message and msgtype=m.text
• (edited) annotation
• implement design, dark theme
• add avatars to pills
• make pillifying, link previewing update on edit
• test locally with eriks flag enabled server support locally and see that we don't display the fallback content when the server already aggregated/replaced the event while back-paginating.
• parse html to markdown (easy)
• re-apply markdown (easy-ish, can copy-paste code from MessageComposerInput::handleReturn probably) Pills should be markdown links and then ran through the markdown parser.
• tab completion

[bwindels]

Tracking undo support for new composer in #9831 as this issue has been in progress forever already.