Add Wikidata to catalogue graph (LoC)

[StepanBrychta]

Determine which Wikidata concepts are mentioned by LoC (or vice versa) and add the corresponding Wikidata concepts to the catalogue graph.

There are at least two ways of achieving this:

• Iterate through the LoC dataset and find references to Wikidata concepts
• Run a SPARQL query against Wikidata to find references to LoC concepts

We should first determine which of these ways is more appropriate (easier to implement).