fix(backend): Foute entity-structuur van leerpaden verbeterd.

Ook testen geschreven voor LearningPathRepository en LearningObjectRepository.

backend/tests/test-assets/learning-objects/learning-object-example.d.ts

@@ -0,0 +1,7 @@
+import {LearningObject} from "../../../src/entities/content/learning-object.entity";
+import {Attachment} from "../../../src/entities/content/attachment.entity";
+
+type LearningObjectExample = {
+    createLearningObject: () => LearningObject,
+    createAttachment: {[key: string]: (owner: LearningObject) => Attachment}
+};