Summary: Modernize Ponder edges to subclass `PhabricatorEdgeType`. Largely based on D11045.
Test Plan: I couldn't actually figure out how to get these strings to show up anywhere.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Krenair, chad, epriestley
Differential Revision: https://secure.phabricator.com/D11083
