R1:623aaf488dc3

Consolidate "Subscribers" Herald field value logic

Summary:
Ref T8455. The Herald code in general isn't nearly as modular as it should be, and the subscriber code particularly has some legacy cruft. This is making it fragile and causing the issue described in T8455.

Currently, each Herald adapter has essentially identical code which it uses to determine which users are subscribed to an object. Instead, share code between object types.

I removed "explicitCCs":

- The value was always identical to doing…