Make Flags policy aware

Summary:
Ref T1809. Ref T603. Ref T3599. Makes flags policy aware.

This change reduces the utility of flag search/browse; the next change will switch it to ApplicationSearch to restore utility. Representing all that ordering in terms of cursor paging is also a giant pain.

Test Plan: Viewed Differential, Flags, etc. Grepped for all PhabricatorFlagQuery callsites.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603, T1809, T3599

Differential Revision: https://secure.phabricator.com/D6746