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 implements a CustomField version of the "Title" field.

(There are no links to this in the UI.)

{F115353}

{F115354}