add message for further results if filters are on

[joaquinvanschoren]

E.g. if you search for dataset 'convex' it is not shown by default because it is 'in preparation' It would be good to show a message like '3 more results that don't match current filters'

[PGijsbers]

For optimization reasons, I think this is probably only a good idea if the search query returns no (or few) results.