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