R1:7cab903943f5

Migrate Differential revision edges to use modern `EdgeType` subclasses

Summary: Modernize Differential edges to subclass `PhabricatorEdgeType`. Largely based on D11045.

Test Plan: From previous experience, these changes are fairly trivial and safe. I poked around a little to make sure things looked reasonably okay.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, Krenair, epriestley

Differential Revision: https://secure.phabricator.com/D11074