Make "Add Subscribers" and "Remove Subscribers" Herald actions modular
Summary: Ref T8726. Converts these actions to be modular. No real surprises in this change.
- Wrote some rules.
- Migrated them forward.
- Used a bunch of these rules.
Reviewed By: btrahan
Subscribers: joshuaspence, epriestley
Maniphest Tasks: T8726
Differential Revision: https://secure.phabricator.com/D13699