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/user.entity.ts

@@ -1,7 +1,7 @@
 import { Entity, PrimaryKey, Property } from '@mikro-orm/core';
 
 @Entity({abstract: true})
-export class User {
+export abstract class User {
     @PrimaryKey({type: "string"})
     username!: string;