Add public activities

[nicksellen]

Closes #2583 Frontend PR: https://github.com/karrot-dev/karrot-backend/pull/1241

What does this PR do?

I did a video walkthrough of how it's working so far:

https://user-images.githubusercontent.com/31616/191973825-de481fb7-87f7-4a64-b824-7e2f919a7c4d.mp4

... and a bit I missed out from the first one ...

https://user-images.githubusercontent.com/31616/191976794-1208e9e8-dd6e-422f-9b90-aaa85f19fc2d.mp4

Looking for small detailed bits of feedback :)

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?)