Course and module management

## 😒 Background:
**User Story**: As a student, I would like to be able to add and organize courses and modules so that I can structure my studies.

* Courses / modules (technical name: folders) can be created at the top level of the data structure
* Modules can be created at the top level of the data structure
* Courses can be created in modules

_Example of a structure:_

- **Modules: Fundamentals of Computer Science**
- **Course: Introduction to Programming**
- Note: Variables and Data Types
- Note: Loops and Conditions
- **Course: Data Structures and Algorithms**
- Note: Arrays and Lists
- Note: Search and Sorting Algorithms
- **Course: Computer Architecture**
- Note: CPU and Memory
- Note: Machine Language

## 🤔 Describe the solution you'd like:
There is already a “type” field in the “Resource” database table. The type “course” or “module” should be saved in this field.

The API request (POST) contains the following information:

* children: string (mandatory; where should the resource be created; empty string if at root level)
* type: string (mandatory; what type of resource should be created; only “course” or “module” allowed)
* Header: JWT (authentication required; must be evaluated)