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