Summary: Modernize Pholio edges to subclass `PhabricatorEdgeType`. Largely based on D11045.
Test Plan: Attached a mock to a task, observed the expected comment in the transaction view (both ways).
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11113
