Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions
     * sms: px to rem, added background size definitions, removed unnecesary css calcs
     * responsive.css: adjusted mediaqueries to do not target desktop screens
     * added @2x assests for sms
Merge pull request #8444 from basiclines/responsive-ui

Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions