R1:102ea3cfa41d

Replace Differential Edit controller with EditEngine-driven EditPro controller

Summary:
Ref T11114. This replaces the old edit controller with a new one based entirely on EditEngine.

This removes the CustomFieldEditEngineExtension hack for Differential, since remaining field types are fairly straightforward and work with existing EditEngine support, as far as I can tell.

Test Plan:
- Created a revision via web diffs.
- Updated a revision via web diffs.
- Edited a revision via web.
- Edited…