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 immediately.

Test Plan: Browsed Config.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7185

Differential Revision: https://secure.phabricator.com/D11764