Modularize Herald Diffusion pre-commit content fields
Summary: Ref T8726. The gruntwork part of this is finally over.
- Made a huge rule with every field.
- Applied migration.
- Verified the rule was still the same.
- Pushed a bunch of commits and verified transcripts.
Reviewed By: btrahan
Maniphest Tasks: T8726
Differential Revision: https://secure.phabricator.com/D13601