Implement PolicyAwareQuery for triggers

Summary:
Ref T6881. I tried to cheat here by not implementing this, but we need it for destroying triggers directly with bin/remove destroy, since that needs to load them by PHID.

So, cheat slightly less. Implement PolicyAware but not CursorPagedPolicyAware.

Test Plan:

• Used bin/remove destroy to destroy a trigger by PHID.
• Browsed daemon console.
• Ran trigger daemon.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11445