Bug 867916 - Adds promo-images as background-images instead on <img> 

This replaces #827 because i had some difficulties rebasing after the big playdoh-upgrade.

* Adds a .loaded class to the html-element so that non visual images isn't loaded instantly
* Extracted promo css rules to home-promo.less
* Added some structure in sized images in /media/img/home/promo/
* Extends Mozilla.Pager with hide-with-hidden functionality
* Made the Pager more a11y by showing the corrrect slide on focus