Move DifferentialRevision to application PHIDs
Summary: Ref T2715.
Test Plan: Used phid.lookup and phid.query to load handles. Grepped for PHID_TYPE_DREV.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2715
Differential Revision: https://secure.phabricator.com/D6509