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/language.ts

@@ -0,0 +1,6 @@
+export enum Language {
+    Dutch = "nl",
+    French = "fr",
+    English = "en",
+    Germany = "de"
+}