Modernize Legalpad edge types

Summary: Modernize Legalpad edges to subclass PhabricatorEdgeType. Largely based on D11045.

Test Plan:

1. Created a Herald rule to require legal signatures on all diffs.
2. Created a new diff.
3. 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