R1:0906bf547b9a

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…