Legalpad - make it work for not logged in users

Summary: Adds "verified" and "secretKey" to Legalpad document signatures. For logged in users using an email address they own, things are verified right away. Otherwise, the email is sent a verification letter. When the user clicks the link the signature is marked verified.

Test Plan: signed the document with a bogus email address not logged in. verified the email that would be sent looked good from command line. followed link and successfully verified bogus email address

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran, asherkin

Maniphest Tasks: T4283

Differential Revision: https://secure.phabricator.com/D7930