Link available activities to only show free slots

[nicksellen]

Fixes #2620

What does this PR do?

As @dwaxweiler described in the issue, makes more sense to link to a view that shows activities in your favourite places that actually have free slots.

Links to related issues

Checklist

• [ ] added a test, or explain why one is not needed/possible...
• [ ] no unrelated changes
• [ ] asked someone for a code review
• [ ] joined #karrot:matrix.org
• [ ] added an entry to CHANGELOG.md (description, pull request link, username(s))
• [ ] tried out on a mobile device (does everything work as expected on a smaller screen?)