Config - add an option to lock policy settings
Summary: Fixes T6947
Test Plan:
locked people.create.user and noted the UI only showed a link to the existing policy with no way to edit it.
tried to set the config to all the various bad things and saw helpful error messages telling me what I did wrong.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6947
Differential Revision: https://secure.phabricator.com/D11358