Give workgroup admins permissions to delete posts in their workgroups

Closes None

What does this MR do?

This MR implements the functionality to let workgroup admins delete posts in their workgroup. This was before implemented improperly.

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

Pretty much, I wrote a test.

Links to related issues

Here is the hotfix, which removed the previous functionality. !1483 (merged)

How to test

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

1. Checkout branch locally
2. Give a foodsaver admin rights for a workgroup and add them to another
3. They should be able to delete posts of anyone in their workgroups and only their own posts in other workgroups

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

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