[FEATURE] refactored to new api

__Tasks:__
- [x] refactor all REST-Controllers to meet the new API conditions
- [ ] gather feedback for my changes 

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | [none]
| Doc           | none
This seems to be something that should be abstracted in the Collection class -- Doctrine `Collection` classes have a method ``->contains($element)`` -- would that not work here?