Support Ferret engine for searching users

Summary:
Ref T12819. Adds support for indexing user accounts so they appear in global fulltext results.

Also, always rank users ahead of other results.

Test Plan: Indexed users. Searched for a user, got that user.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18552