Allow users to search for signatures across documents

Summary:
Ref T3116. You can already search for sigatures on a specific document, but allow them to be searched across documents too.

In particular, this lets users answer questions like "Which of these 5 documents has alincoln signed?" / "Has alincoln signed all the stuff I care about?" / "who has signed either L5 or equivalent document L22?", etc.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9770