Add license information manually when not given in dump

[DavidBRohrer]

When missing license information in vocabulary data, there should be an option to enter them manually or to retrieve them from BARTOC.org.

For instance, there are no license information in the rdf-file of https://skosmos.bartoc.org/39: https://research.un.org/en/thesaurus/downloads.
But the UNBIS Thesaurus can only be "accessed and used free of charge for non-commercial use, provided appropriate acknowledgement is given as follows:

© United Nations, 2019, http://metadata.un.org/thesaurus/
Information downloaded on [DATE]".

In Skosmos dc:license is supported (https://github.com/NatLibFi/Skosmos/wiki/Data-Model#dublin-core-properties) but not listed in the vocabulary-specific configuration: https://github.com/NatLibFi/Skosmos/wiki/Configuration#vocabulary-specific-configuration.

[nichtich]

Requires NatLibFi/Skosmos#1416 or merging of license information into the vocabulary dump.