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

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)