Use ApplicationSearch to search for Legalpad signatures

Summary:
Ref T3116. Currently, document signatures are just in a big list that you can't search through.

• Make it easier to check if a specific user has signed.
• Restrict this UI to users who have edit permission on the document (roughly, you need to be a document manager to see the full signature list).

(It's currently possible to generate a Dashboard panel using this query, but it will just throw an exception. I'm going to leave it like that for now, we might reasonably expose some "view signatures across doucments" UI later so someone can quickly check if a user has signed 5 documents or something.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9765