Add indicator whether more results exist to Searchable{Event|Series} element

[LukasKalbertodt]

We are using react-select and just fetch from specific search APIs. Those APIs have a hardcoded maximum number of items they return. We just show those. We have no indicator in the UI whether there are more events. Us lazy programmers probably don't see the problem because we would just refine the search query and not bother scrolling down, but normal user might just scroll through the whole list and wonder why their event isn't there, assuming that what they see is all there is.