Make "ADD_CC" and "REMOVE_CC" available as "standard" Herald effects

Summary:
Ref T8455. Begins consolidating the code for applying these effects:

• Makes Add/Remove subscribers a standard effect, and uses it in Pholio.
• This includes the "don't re-subscribe users who have explicitly unsubscribed" logic from Differential in the standard effect. I think this rule is always desirable.
• This adds new filtering of invalid PHID types to resolve the arc diff issue in T8455 once Differential uses this standard effect.
• Added "Remove Subscribers" to MockAdapter in order to test that it works.
• Relabeled "CC" in Pholio to "Subscribers" for consistency.

Test Plan:

• Created several rules which add subscribers to (and remove subscribers from) mocks.
• Updated mocks, changing properties and adding and removing subscribers.
• Observed transactions applying and aggregating properly.
• Observed add/remove rules each working correctly.
• Observed the "don't re-add unsubscribed users" condition acting on subscribers who had previously been added but explicitly removed/unsubscribed.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8455

Differential Revision: https://secure.phabricator.com/D13179