R1:83d1e3edb59b

Modernize Legalpad edge types

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…