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 nonstandard custom fields ("Blame Revision", "JIRA Issues").
• Created a revision via CLI.
• Updated a revision via CLI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17054