R1:e5b402d13f76

Lock all reply-handler options in the upstream, plus cookie prefix

Summary:
Ref T7185. These settings shouldn't be unlocked anywhere. Specifically:

- `reply-handler`: These are on the way out.
- `reply-handler-domain`: Also hopefully on the way out; locked because a compromised administrator account can redirect replies.
- `phabricator.cookie-prefix`: Not dangerous per se, but an admin could have a hard time fixing this if they changed it by accident since their session would become invalid…