Allow users to search for signatures by name and email substrings

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