[fix bug 932371] Update copy on Lightbeam site

Updates copy as per bug [932371](https://bugzilla.mozilla.org/show_bug.cgi?id=932371)

Also took some time to fix up a few things:

* Improved semantics a bit, removing some presentational pieces of markup and CSS classes.
* Fixed responsive alignment issues with the first main image on the Lightbeam home page
* Fixed up CSS formatting and indentation.
* Stopped all the links from opening new windows.

There's still more that can be improved on this page, but that's all I had time to do for now.