Mapping Navigator: Prioritize reverse mapping of selected concepts higher

[stefandesu]

If we have selected A on the left and B on the right, and there's a mapping B -> A, it should be shown higher in the list of mappings in Mapping Navigator (possibly right under existing mappings A -> B).

This issue is easily implemented right now, but when implementing proper pagination for Mapping Navigator (#367), it needs to be reconsidered and possibly implemented in jskos-server as a sorting option.