Allow feature flags to be disabled on an actor-by-actor basis

Issue corresponding with Allow feature flags to be disabled on an actor-... (!98575 - merged).

Description

Prior to this change, if a flag has been globally enabled, then disabling it for an actor would not do anything, making it impossible to have flag opt-outs.

This change maintains a synthetic disabled flag alongside each active flag, and checks them to see if the actor is marked as in the disabled list first. If so, then we return disabled, even if the feature is globally enabled, allowing individual opt-outs.