Provide a simple read-only maintenance mode for repositories

Summary:
Ref T13393. While doing a shard migration in the Phacility cluster, we'd like to stop writes to the migrating repository. It's safe to continue serving reads.

Add a simple maintenance mode for making repositories completely read-only during maintenance.

Test Plan: Put a repository into read-only mode, tried to write via HTTP + SSH. Viewed web UI. Took it back out of maintenance mode.

Maniphest Tasks: T13393

Differential Revision: https://secure.phabricator.com/D20748