Modularize Herald Diffusion pre-commit content fields

Summary: Ref T8726. The gruntwork part of this is finally over.

Test Plan:
  - Made a huge rule with every field.
  - Applied migration.
  - Verified the rule was still the same.
  - Pushed a bunch of commits and verified transcripts.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13601