Made the embedder work again with the staticmedia app

Added the concept of a release URLs for staticmedia.  Added a very
simple "bootloader" for CSS files which is a simple @import statement.

Added embedder CSS.  Changed image URLs to be relative to the the
/css/ dir, not the /release/public/ dir.  This works with the new CSS
bootloader system.

Updated our documentation for media.  Since release URLs need a bunch
of explanation I figured now was a good time to do this.