Summary: Modernize Legalpad edges to subclass `PhabricatorEdgeType`. Largely based on D11045. Test Plan: # Created a Herald rule to require legal signatures on all diffs. # Created a new diff. # Saw the transaction string appear correctly. I wasn't able to check the inverse transaction because there is none. Also, I couldn't see any text on the feed (presumably, transactions authored by Herald do not generate feed items) Reviewers: epriestley, #blessed_reviewers Reviewed By: epriestley, #blessed_reviewers Subscribers: Krenair, chad, epriestley Differential Revision: https://secure.phabricator.com/D11082