Begin adding "pro" modular transaction fields to Differential

Summary:
Ref T11114. Currently, all of Differential is extremely custom CustomFields. I want to back away from that somewhat and leverage more EditEngine / ModularTransactions infrastructure.

This allows EditEngine, ModularTransactions, and CustomFields to coexist in an uneasy peace. The "EditPro" controller applies a different edit than the CustomFields do, but everything works out in the end. I think.

Hopefully the horrible mess I am creating here will be short-lived.

Test Plan:

• Edited a revision with the normal editor.
• Edited a revision with the pro editor.
• Created a revision with arc diff.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17044