webpack: Generate CSS with relative paths
webpack: Generate CSS with relative paths

The file loader must not return absolute path, instead the return the
relative file path. The CSS extraction plugin will use a different
public path than global. Therefore `url(...)` will use a CSS-file
relative path.

Note: CSS bundles *must* be compiled into the output top directory
otherwise the URLs will be wrong. This is because the plugin does use an
empty public path. It cannot (yet?) use the bundle output path to
generate a bundle specific public path.

jgraichen/mnemosyne-server,jgraichen/mnemosyne-server,jgraichen/mnemosyne-server