Draft: Move RegionsAdmin to vue + REST

Closes #1412, #1414, #1416, #1417, #1181

What does this MR do?

Code cleanup and refactoring.

• Move functionality of MethodsXhr to API
• Move regions admin page to vue
• Remove unused code (among other things dynatree)

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

Quite confident, but larger cleanups should be reviewed thoroughly.

Screenshots (if applicable)

Open TODOs

• Add GOALS functionality
• Add tests
• Improve data checks

Checklist

• 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 in the release notes to /release-notes/YYYY-MM.md
• Once your MR has been merged, you are responsible to create a testing issue in the Beta Testing forum: https://foodsharing.de/region?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.