Draft: Refactor region admin module

Part of #1181 (closed) Closes #1412 (closed), #1414 (closed), #1416 (closed), #1417 (closed)

What does this MR do?

TODO:

• show working groups in the same tree or in a separate list?
• moving regions: drag and drop in the tree?
  • working groups need to lose their functions when being moved
• changing region admins: tag select or avatar list?
• frontend validation

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

Links to related issues

How to test

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• added to the next milestone (see https://gitlab.com/foodsharing-dev/foodsharing/-/milestones, unless it has a "for:Dev" label)
• added an entry to CHANGELOG.md
• added a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in the Beta Testing forum: https://foodsharing.de/?page=bezirk&bid=734&sub=forum. Please change the MRs label to "state:Beta testing".
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...) how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text in German