Legalpad - add a view signatures page

Summary: ...needs to add a LegalpadDocumentSignatureQuery class to get this done, which is also re-deployed everywhere we were issuing raw queries. Ref T3116.

Test Plan: viewed some signatures. Verified color and footer icons showed up how I wanted them to.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D7986