Allow configuration of Differential custom fields

Summary: Ref T2222. Ref T3886. This is a little early for general use, but the message parse/generate stuff requires CustomFields and FieldSpecifications to be closely aligned, so this provides at least a plausbile approach for any installs that run into trouble.

Test Plan: Viewed config; reordered fields.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222, T3886

Differential Revision: https://secure.phabricator.com/D8450