Forum follower updates migration for !1191

What does this MR do?

• Create a command that inserts forum follower entries for all participants of conversations that haven't got an entry yet. This entry has bell notification on and mail notification off and should reflect the default behaviour before !1191 (merged).

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

quite

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))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings