The site-icons spritesheet has already become unwieldy - it's almost
1MB, is mostly rarely-needed icons, and needs to be fully replaced and
re-downloaded whenever a new icon is added. With HTTP/2 now being widely
supported, spritesheets seem to be mostly obsolete, and I probably never
should have done it that way in the first place.
This commit changes over to simply using individual icon images, and
rebuilds the CSS file whenever new icons are downloaded. This new CSS
file will probably be somewhat large, but should gzip extremely well.
This probably still needs some work to support cache-busting on the CSS
file.
Deimos