Add user omnipotence

Summary:
Daemons (and probably a few other things) need to make queries without having a real user. Introduce a formal omnipotent user who can bypass any policy restriction.

(I called this "ominpotent" rather than "omniscient" because it can bypass CAN_EDIT, CAN_JOIN, etc. "Omnicapable" might be a better word, but AFAIK is not a real word.)

Test Plan: Unit tests.

Reviewers: vrana, edward

Reviewed By: edward

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D5149