Move more CSS files into new directories.
Review Request #5862 — Created May 22, 2014 and submitted
This moves a few more CSS files out of the root and into organized
directories.The DnD uploader component's stylesheet has been moved into ui/.
The retina mixin has been moved into mixins/.
The icon definitions have been moved into assets/.
Tested various pages. Saw the icons. Saw the DnD uploader.
Built a package successfully.