Summary: Modernize Dashboard edges to subclass `PhabricatorEdgeType`. Largely based on D11045.
Test Plan: Attached a panel to a dashboard, observed the expected comment in the transaction view (both ways).
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D11114
