Introduce a choice box when creating a new thread to lower email load especially from ambassadors

Part of #64 (closed)

What does this MR do?

It introduces a choice to send mail for a new forum thread in unmoderated forums.

There is a problem with spacing and position of these radio buttons as I would like them to be place above the send button.

Tbh I'm not interested in diving too deep into this but maybe we figure it out together? ~"help wanted"

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

Should work. Jo tested it quite intensively with moderated forums and so on Caluera tested tests and visibility only in not-moderated forums locally

Links to related issues

• #64 (closed) Think about a way to reduce the amount of emails

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