Mostly move inline editing to DiffInline
Ref T12616. This doesn't pull over everything (some UI feedback didn't make it yet, and you can't cancel + undo cancelling edits yet) but editing comments technically works.
This is a little shaky, but feels less shaky than every other approach I've tried, so I think I'm finally on a reasonable track here.
Test Plan: Edited some inline comments.
Reviewed By: chad
Maniphest Tasks: T12616
Differential Revision: https://secure.phabricator.com/D17887