Legalpad - allow for legalpad documents to be required to be signed for using Phabricator

Summary: Fixes T7159.

Test Plan:
Created a legalpad document that needed a signature and I was required to sign it no matter what page I hit. Signed it and things worked! Added a new legalpad document and I had to sign again!

Ran unit tests and they passed!

Logged out as a user who was roadblocked into signing a bunch of stuff and it worked!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7159

Differential Revision: https://secure.phabricator.com/D11759