feat(backend): Entities toegevoegd

Entites Class, Class Join Request, Class Invitation, Assignment, Group en entities om de Dwengo-leerinhoud te modelleren, toegevoegd.

backend/src/entities/users/student.entity.ts

@@ -1,6 +1,9 @@
 import {User} from "./user.entity";
-import { Entity } from '@mikro-orm/core';
+import {Collection, Entity, ManyToMany} from '@mikro-orm/core';
+import {Class} from "../classes/class.entity";
 
 @Entity()
 export class Student extends User {
+    @ManyToMany(() => Class)
+    classes!: Collection<Class>;
 }