API for event status response

Part of #798 (closed)

What does this MR do?

New REST endpoint for confirming / declining event invitations. It is used by the panels on the dashboard and in the header of the event's page.

TODO: add unit tests

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

quite sure, seems to work

How to test

1. Checkout branch locally
2. Login as foodsaver
3. Go to any region and create a new event
4. On the dashboard, answer to the event
5. Go to the event's page and check that the answer was saved
6. Answer again and check that it is saved again

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 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