Auth - add "manage providers" capability
Summary: Ref T6947.
Test Plan: toggled setting in application settings and changes stuck. set policy to admin user a only and could not add a provider as a admin user b.
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6947
Differential Revision: https://secure.phabricator.com/D11356