Convert stylesheets to scss

So that we can use image-url
Fix path.

Seems clearer and more robust to specify it relative to rails root.
Add stylesheets to precompilation list
Fix image link.

Previously this pointed to a non-existent image. That's a problem with
the asset pipeline, so point it to what I assume was the intended
target.
No need to symlink anymore.

Superceded by asset pipeline.