Streamline Legalpad signature workflow

Summary:
Generally reduces friction, standardizes, and simplifies this workflow. Particularly, this removes "address" and "phone", which I think we can wait for user demand for.

For logged-in users, we just always use their primary email.

Test Plan: See screenshots.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9735