Make "Add Subscribers" and "Remove Subscribers" Herald actions modular

Summary: Ref T8726. Converts these actions to be modular. No real surprises in this change.

Test Plan:

• Wrote some rules.
• Migrated them forward.
• Used a bunch of these rules.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13699