Support indirect mappings as mapping recommendations

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For instance via Wikidata (#170) but other concept schemes can be used for indirect mappings as well (e.g. IxTheo-DDC-GND via DDC). Requires https://github.com/gbv/jskos-tools/issues/5 and https://github.com/gbv/jskos-server/issues/35 and some way to configure.

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Examples from https://doi.org/10.5281/zenodo.3257136 using Wikidata as Hub:

• Example 0 (RVK)
• Example 0 (DDC)
• Example 3b: Emotion

We don't support (this would be another issue)

• OR-groups (e.g. "Sensory Perception OR Movement OR Emotions OR Physiological drives")
• nested groups (e.g. "(Philosophy OR Theory ) ⊓ Law")