Use relative paths for static-root / css-root - fixes #134

Using paths like /static/wagtailadmin/images/ in our SCSS breaks when a STATIC_URL
other than /static/ is in use - for example, when static assets are hosted on an
external domain. However, django-compressor consistently puts the compiled CSS
at ${STATIC_URL}/CACHE/css/ regardless of operation mode, so we can reliably use
../../ to reach the correct STATIC_URL.