[WIP][ticket/12393] Allow users to quickedit their posts

This only has a basic set of features right now. It does not include any
posting buttons. Triggering quickedit, cancelling quickedit, submitting the
edited post, and switching to the full editor works in general.
However, polls get deleted upon editing the first post. Other features haven't
been tested yet to be able to judge to what extent they are working after
quickediting a post.

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12393