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
7cab903943