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.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6947

Differential Revision: https://secure.phabricator.com/D11356