Expand concepts in mappings with labels

[nichtich]

Mappings are usually stored without concept labels, because these labels are better stored independently stored as part of the concepts. Cocoda and other applications load concept labels with subsequent requests. This does not work for downloading in CSV/TSV.

If the jskos server instance also hosts the concepts beeing mapped, their labels (and possibly other fields) could be added to the mappings. Otherwise a dedicated converter service is needed to combine mappings with labels and convert it to CSV or other formats.

Current use case:

• ULBB to BK mappings

Possibly related issue #222 and #110