Modernize Ponder edge types

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