R1:65bc2b1ac53d

Implement "Pro" version of revision editor, with one field

Summary:
Ref T3886. I spent a few hours trying to make `DifferentialFieldSpecification` extend `PhabricatorCustomField` so I could be more blunt in my approach here and just swap the whole thing over in one go (more or less like I did with Maniphest) but we have a ton of custom fields and things felt really shaky and the change was enormous and hard to keep track of.

Instead, I'm going to do this more gradually and go field-by-field. This…