Do not include work group admins into ambassador number in region forum statistics

Closes #778 (closed)

What does this MR do?

The number of ambassadors atop a regions forum does not include the workgroup admins anymore.

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

Very, it works well locally and is only a small change in the statistics.

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Login as orga
2. Click on the gears and then on 'edit region'
3. Set one region as parent region of another
4. Set at least one ambassador for both regions
5. Run ./scripts/docker-compose run --rm --no-deps app php -f run.php Stats bezirke
6. Go to the forum of both regions
7. See if the number of ambassadors in the statistics at the top equals the number of ambassadors in a region and its subregions

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