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
I think it's better to show the button even to people that cannot access the option, so that they can at least discover it and ask for the correct permission if they think they should be allowed to use it.