Support uploading mappings in CSV format

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This requires additional information about the CSV table such as which concept schemes the concepts belong to and how to build URIs from notations.

The conversion step has already been implemented here in Perl:

• https://github.com/gbv/cocoda-mappings/blob/master/csv/lib/GBV/ConceptScheme.pm
• https://github.com/gbv/cocoda-mappings/blob/master/csv/csv2jskos.pl

and to some degree in JavaScript:

• https://github.com/gbv/wikidata-jskos/blob/master/lib/ConceptScheme.js

The latter script should better be moved to jskos-tools anyway.

Labels are ignored but may later be used for error checking.

Because import is error-prone, the import may first go into a local staging storage to let users accept/abort the import (e.g. "detected 12 mappings between 12 concepts from RVK to 10 concepts from BK. Import to local mappings yes/no?"). Most common errors to detect and give feedback during import:

• missing column labels fromNotation, toNotation
• unknown column labels (can be ignored but better give a warning)
• invalid notations (requires information about syntax of valid notations)

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This should first fully be implemented and tested in jskos-cli.