Make DrydockLease a policy-aware object

Summary: Ref T2015. DrydockLease predates widespread adoption of policies. Make it -- and its query -- policy aware.

Test Plan: Browsed leases from the web UI. Grepped for callsites.

Reviewers: btrahan

Reviewed By: btrahan

CC: hach-que, aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D7826