Ensure absolute URL paths start with /

[nicksellen]

Fixes #2618

What does this PR do?

The public activity link was being generated with a missing /, which sort of worked, but I think some programs didn't recognise it as a link or something. This ensures we always generate links with that / in place.

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?)