Index users in search results.

Summary:
Add users to the search results. I need to follow this up with a patch to make
the search results stop being terrible. I'll do that.

Test Plan:
Searched for users, ran "reindex_all_users.php"

Reviewed By: jungejason
Reviewers: tomo, jungejason, aran
CC: aran, jungejason
Differential Revision: 508