R1:45d61b711060

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…