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