Actually enforce auth.lock-config

Summary: Forgot to post this after D20394. Fixes T7667.

Test Plan:

• Edited some providers with the config locked and unlocked.
• Opened the edit form with the config unlocked, locked the config, then saved, and got a sensible error:
  

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T7667

Differential Revision: https://secure.phabricator.com/D20645