Modularize all Herald pre-commit ref fields

Summary: Ref T8726. Just (pre-commit content) one more left.

Test Plan:

• Created a big rule with every field.
• Migrated it.
• Verified the rule was still the same.
• Pushed a bunch of changes and reviewed the transcripts.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13600