People - add application policy on user creation
Summary: Ref T6947.
Test Plan: made the setting say only admin user a and noted admin user b lost access
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T4137, T6947
Differential Revision: https://secure.phabricator.com/D11357