Adding in SVG icons, renaming arrow images, adding in mobile sprite, changing right sidebar icon

Here's what's different:
- PNG icons for Meta, Brickipedia, Customs, and Admin replaced with PNGs to look better on high-pixel-density (such as Retina) displays (note that Dev, Cuusoo, and Stories still need SVGs)
- High-res arrow images renamed so the server stops loading the old ones (currently they have the same filenames as the old versions)
- Removed mobile-expand.png and added in mobile-icon-sprite.png, changed the right sidebar button on smaller displays so it uses the mobile sprite icon and not the mobile-expand one