Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions
     * shared/style and shared/style_unstable css ready for bigger asstes (background-size declarations mostly)
     * added @2x assests for all the shared branding and style folders
Merge pull request #8380 from basiclines/responsive-ui-shared

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