[REFACTORING] Refactored category api

Refactored category api. 
The parent categories of the full response now also contains their children.

__Tasks:__

- [x] Refactor API
- [x] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | none