Accept absolute css paths

An absolute url in a design does not load correctly in the editor.
Design:
```json
{
  "config": {
    "css": ["http://example.com/some/url/style.css"]
}
```

With the current solution, this results in `/designs/bootstrap/http://example.com/some/url/style.css`.
Assets should be linked using `./style.css` or `style.css`. Otherwise they should be handled as absolute urls.