Move buddy requests to REST

Part of #798 (closed), might solve #847 (closed)

What does this MR do?

Creates a new REST endpoint for sending buddy requests and makes the frontend use that endpoint.

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

Quite sure, the behavior should be the same as before. I added some unit tests for the API.

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. Go to another users profile and send a buddy request -> you should see a confirmation message
4. Login as that other user
5. You should see a bell notification
6. Accept the buddy request -> you should see a confirmation message 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
• 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 (not necessary for refactoring)
• 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.