Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions
         * browser: px to rem, added background size definitions, removed unnecesary js function to show lateral panel
         * added @2x assests for browser
Merge pull request #8489 from basiclines/responsive-ui

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