R1:8e3396ce21a9

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