R1:852ecc21023f

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…