Notification for being accepted in a working group

Closes #808 (closed)

What does this MR do?

Adds a bell notification for a user who is accepted to a workgroup. I used the opportunity to move the Xhr backend function to a new REST controller.

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

Very sure

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. Login as foodsaver
3. Apply for a work group
4. Login as userbot
5. Go to that work group and accept the application
6. Login as foodsaver again
7. You should see a bell which leads you to the group's page

Screenshots (if applicable)

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
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add 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 Beta Testing Repo:
  • 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

Wenn eine Bewerbung für eine Arbeitsgruppe angenommen wird, erhält die aufgenommene Person jetzt eine Glocken-Benachrichtung darüber.