Modernize Dashboard edges

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