Add Header to similar clusters

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@mutwil which page ? There is a sortable header in the Similar clusters panel on the cluster page ...

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The only place I can spot where the cluster similarity is shown has a header ;-)

There is a more general issue with paginated tables which cannot have a sortable header similarly to normal tables (it would only sort the visible elements which is incorrect, here a round trip to the server would be required). To this end I have omitted a header entirely, but I could include a non-sortable header there.