Improvements to mobile landing page

The first round of fixes to the mobile landing page /en/mobile/ for HTML5Rocks.

Updates include:
* Updated copy, including removing many old and outdated links, and added new, more relevant content
* New landing page image
* Removed dark background for landing page
* Added PageSpeed insight service to landing page
* Made disabled buttons appear more disabled [site wide fix]
* Fixed issue where only 1/2 of screen is used on some mobile devices [site wide fix]