Fixes bug 1073591 - Added a simplified UI to Super Search.

@peterbe r?

Screen shots: 
 * Advanced mode - https://bug1073591.bugzilla.mozilla.org/attachment.cgi?id=8524381
 * Simple mode - https://bug1073591.bugzilla.mozilla.org/attachment.cgi?id=8524382
This looks very much like a setting. If you put it in `settings/base.py` then it gives you an opportunity to write a comment that explains that it's the default fields in the simple super search UI.