Move Differential's read-only fields to the extensible field schema

Summary:
Move additional fields (which rely on loading handles) to the extensible field
classes and out of hardcoding in the controller.

Depends on D807.

Test Plan: Viewed, edited, and hit conduit for revisions.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason, epriestley
Differential Revision: 808