Show signatures in a table instead of an object list

Summary:
Ref T3116. Since this UI was written we've moved away from footer icons and made tables work better on mobile. This seems reasonable to use a pure table for. I've also reduced the number of required fields here. Use a table and make this UI accessible.

The "Restricted External Account" stuff is T3732, which I'll tackle next.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9766