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.