Modularize all DifferentialRevision fields

Summary: Ref T8726. Make all the DifferentialRevision stuff modular.

Test Plan:

• Created a rule with all fields.
• Ran upgrade.
• Saw all fields preserved with new modular versions.
• Used test console to run rule with all fields, verified field values as broadly sensible.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13598