Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions
                 * system [all components but the ones already landed]:
                   px to rem, added background size definitions.
                 * added @2x assests for system
Merge pull request #8710 from rexboy7/responsive-ui

Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and reso...