Update Legalpad for EditEngine

Summary: Updates Legalpad to use EditEngine, paving the way for transaction comments. Spooky.

Test Plan:

• New Document
  • Require signing, Corp - see fail
  • Require signing, Noone - see fail
  • Require signing, Ind - get asked to sign
• Edit Document

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17862