Provide API route to export metadata index in CKAN format

This is the first API route to export our metadata index in a standardized format.

Initially, we implement only package_search to allow ckan harvesters to get all datasets in an reasonably economic fashion. Querying single datasets might be implemented at a later point.

Also, the only allowed export format on this route is json.

• Create route ckan/package_search delivering json formatted responses.
• Translate ScoredDataset into valid CKAN