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
