Improvements for dublin core to schema.org mapping

We have to overthink the mapping from dublin core to schema.org.

Known problems:

The provider field should be filled by the information from the harvester and not from the dc field source (source might be linked to related identifiers through.) (see issue unhide-ui#52)
Currently we do not provide @ids only identifier, the uplifter/aggregator corrects for this, it would be nice to do it such in the first place.