R1:ffe0765b50c5

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…