Mostly move inline editing to DiffInline

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17887