Summary:
Ref T3116. In the case of anonymous signers, there's no way to do a quick way to check if someone has signed a doc since you can't query by their (nonexistent) external account ID.
Move "name" and "email" to first-class columns and let the engine search for them.
Test Plan: Searched for signatures with name and email fragments.
Reviewers: btrahan, chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T3116
Differential Revision: https://secure.phabricator.com/D9776
