Move Differential's remaining field views to extensble field schema

Summary:
Move all the rest of the fields into the custom field schema, for revision
views.

I left a couple of stubs in here (willWriteRevision, didWriteRevision) since I'd
planned to do edits here too, but this diff is sort of big-ish already. I'll do
all the edit fields in the next revision.

Depends on D808.

Test Plan: Viewed, edited and conduit'ed some revisions.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason, epriestley
Differential Revision: 809