[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
`cget` stands for collection get, and returns multiple items. It's a convention defined by the FOSRestBundle: https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/Resources/doc/5-automatic-route-generation_single-restful-controller.md