R1:1fb39a20d3fa

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