When loading project membership to evaluate the "Subscribers" policy, use the ominipotent viewer

Summary: See PHI448. Ref T13106. The current implementation here can end up in an infinite stack if, e.g., a project uses "Visible to: Subscribers".

Test Plan: Will push.

Maniphest Tasks: T13106

Differential Revision: https://secure.phabricator.com/D19226