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
