R1:ee4c7268fe23

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…