Add basic NUX support to SearchEngines

Summary:
This is just putting a hook in that pretty much works. Behavior:

• If you visit /maniphest/?nux=true, it always shows NUX for testing.
• Otherwise, it shows NUX if there are no objects in the application yet.

Test Plan: {F1031846}

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14828