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
That's how a lot of websites worked when I started writing web apps back in the late 90s. The reason for that was because we often used Basic Authentication. To get the prompt for username+password (often of http which is mindboggling to think today) you had to get the user to click on something they don't have access to. Often, instead of a link to `/login` it would be a link to `/user/account/dashboard` or something. 

We can bikeshed about this but I'm pretty confident showing the button as a temptation would be breaking a "pattern" we have. For example, we **don't** show "Email: (not visible to you because you don't have the permission)". And we don't show the `<a href="/admin">Admin</a>` link in the footer until you sign in as a superuser. And we don't show "Exploitability report" in the Reports drop-down unless you have the right permission. 

Ultimately it's your decision and I value that more than my own opinion but bear in mind the point that this would be a breakage from a certain pattern of consistency.