Move Differential's simple fields to the extensible field schema

Summary:
Differential has a bunch of display-only fields, implement them all as field
specifications instead of hard-coded fields.

Also add some more documentation and fix redundant string constants in blame
rev/revert plan fields.

Test Plan: Viewed, edited, and hit conduit for revisions.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason, epriestley
Differential Revision: 807