Legalpad - add policy rule for legalpad document signatures

Summary:
Ref T3116. This creates a policy rule where you can require a signature on a given legalpad document.

NOTE: signatures must be for the *latest* document version.

Test Plan: made a task have a custom policy requiring a legalpad signature. verified non-signers were locked out.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D7977