Limit results returned by typeahead query in response to user searches

Summary:
Currently, on secure.phabricator.com, if you type "e" we generate about 600 users and ship them over the wire. This takes ~300ms.

Instead, limit the results to a superset of what the client will actually show.

Test Plan: Ran user typeahead queries, tweaked limit to 1.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3121