feat: draw multiple connections in node link visualization

If there are multiple edges between two nodes, draw multiple connections between these nodes.

This is a draft, what may be considered is:

• a configuration to turn this off and on
• Leo gave an example of a person being both an actor and director. I can make a query for that, but this does not currently result in multiple edges between nodes being returned. To enable that, we would probably need to update the backend / query parsing to allow for this as well. → fixed with e1a5768e

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