R1:a6acedef0b24

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