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 model opt-outs via actor flipper-ids.
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.

Disabling a flag for an actor and opting out are distinct operations.

We also include changes to handle feature flag groups. While we don't
use any ourselves, we do support the manipulation of feature flag groups
through the API, and self-hosted instances may provide their own group
definitions, so we must continue to support this. Currently opting-out
for groups is a NO-OP, but might be supported in the future.

Support for the opt_out/remove_opt_out operations is added to the
REST API.