Differential - make DifferentialDiffEditor into a real transaction editor.

Summary: Ref T6237. This sets us up for some future work like T6152, T6200 and generally cleaning up this workflow a bit. Tried to do as little as possible so not exposing transaction view yet. (Though that timeline is going to be a little funky in the common case of just the lone create transaction.)

Test Plan: made a diff from web ui and it worked. made a herald rule to block certain diffs then tried to make such a diff and saw UI letting me know i was blocked

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6237

Differential Revision: https://secure.phabricator.com/D10869