List of bare data items in collection view should be replaced by a count grouped by category

Context: I was diagnosing issues where daphne gets into troubles when handling requests that take too long to reply and stumbled upon a case where https://debusine.debian.net/debian/developers/collection/debusine:task-history/_/ would not load in 60 seconds...

By default the collection view shows a count of artifacts by category (example) and a full list of bare data items. We should get rid of that full list of bare data items as it creates issues because it's not paginated.

Instead we should just do the same as for artifacts and list the categories of the bare data items and link back to the browse view for details.