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