Make legalpad document list a little nicer for unsignable documents

Summary:
This just cleans things up a little:

• Don't show signature status if the document isn't signable.
• Show "Not Signable" instead of "No One" to make the meaning more clear in this context, where we don't have a "Who should sign:" sort of cue.

Test Plan:

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11834