Modernize Pholio edges

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