R1:d8bba221b52b

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…