Better error message/handling when a forum thread gets removed
When a forum post is deleted it can potentially remove the thread.
Currently, the auth check returns 403 for non-existand threads, make it return 404 instead. To come is properly parsing this in the frontend.
- added a test, or explain why one is not needed/possible...
- no unrelated changes
- asked someone for a code review
- joined #foodsharing-beta channel at https://slackin.yunity.org
- added an entry to CHANGELOG.md (description, merge request link, username(s))