Maps icon + spriting of cats (bug 869390)

This branch converts cat-icons to use a svg sprite which uses cloned paths to save on file-size.

The maps icon is added and I've added small bit of JS to add a necessary class so that mobile and desktop still work in the same way that they do currently (On mobile screen the current category is always grey where on desktop it's blue). If this could be made consistent then that would simplify things a little bit.

Annoyingly due to a lack of background-position-x/y the CSS ends up more verbose. However the net win is 2 less http requests and no delay when hovering before you see the hover state image.