Add a basic search typeahead

Summary:
This needs a bunch of refinement but pretty much works. Currently shows only users and applications. Plans:

• Show actual search results too.
• Clean up the datasource endpoint so it's less of a mess.
• Make other typeaheads look more like this one.
• Improve sorting.
• Make object names hit the named objects as the first match.

Test Plan: Will attach screenshots.

Reviewers: btrahan, vrana, chad

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3110