Enable CORS for GEO label requests

Related to lushc/geoviqua-geonetwork-plugin@69b046ceff (an example XHR using this PR).

This gives us a way to create cross-domain XHRs for requesting a GEO label for a metadata record. It's a bit limited in scope as it'll be restricted to the tutorial's configuration (i.e. what label, feedback and GeoNetwork instances it points to), so a cross-domain request for a GEO label using a GeoNetwork ID won't be that useful. However, I've added a new parameter "metadata" which will just proxy the request to the configured GEO label service. This is used by the schema plugin (see above) to POST the serialized XML of the metadata record currently being viewed and embed the resulting GEO label into the view directly.