Forum fixes

What does this MR do?

Refactors the reactions as well as fixes some functionality around the forum.

Needs production database migration on merge (will remove all existing reactions)

How confident are you it won't break things if deployed?

quite

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 #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))