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/content/attachment.entity.ts

@@ -0,0 +1,16 @@
+import {ManyToOne, PrimaryKey, Property} from "@mikro-orm/core";
+import {LearningObject} from "./learning-object.entity";
+
+export class Attachment {
+    @ManyToOne({entity: () => LearningObject, primary: true})
+    learningObject!: LearningObject;
+
+    @PrimaryKey({type: "integer"})
+    no!: number;
+
+    @Property({type: "string"})
+    mimeType!: string;
+
+    @Property({type: "blob"})
+    content!: Buffer;
+}