reusing existing mappings

[nvdk]

allow reusing the mapping-config from another vocab (all skos-like bodc vocabs are essentially the same)

When creating a mapping for the first time, have the option to import an already existing mapping.
importing the mapping is "best effort": it will not import mappings that are not present in the current dataset. e.g. if a type or predicate is missing it will not be imported.

We can assume that the number of existing mappings is limited, no pagination needs to be provided but a basic search should be available.