Allow Differential custom fields to appear on edit and view interfaces

Summary: Depends on D798. Extends custom fields and makes the vaguely useful:
they can appear on the edit and view interfaces. This does not integrate them
with commit messages yet; that's more complicated but I plan to do it shortly.
Test Plan: Implemented a custom field per P123, it correctly appears on the edit
interface, persists, validates, and shows up when viewing the revision.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, epriestley, jungejason
Differential Revision: 800