Make document signatures visible to only document owners and signers

Summary:
Ref T3116. Currently signatures are visible to anyone, but they should be more private than that. Instead, you can see a signature if:

• It's a signature on a document you can edit; or
• it's your signature.

I'm going to lock down the signatures page a bit in general, but this makes sure that the root policy is correct.

Test Plan:

• Signed a document.
• Viewed signatures of a document.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9764