Make images in Admin Console Retina-friendly

Goals:

1. use SVG images for Admin Console tree menu with auto-fallback to PNG version if SVG image is missing
2. use SVG images inside grids (e.g. item icons)
3. use SVG images in other places (small images on buttons mostly)
4. create Retina-ready toolbar icon sprites + update CSS
5. vertically center selector icons (e.g. user selector, date selector, category selector, etc.)
6. change look of icons to be flat-style, but keep color