feat(backend): ClassRepository & gerelateerde repo's aangemaakt

ClassRepository, ClassJoinRequestRepository en TeacherInvitationRepository aangemaakt.

backend/src/entities/classes/teacher-invitation.entity.ts

@@ -0,0 +1,18 @@
+import {Entity, ManyToOne} from "@mikro-orm/core";
+import {Teacher} from "../users/teacher.entity";
+import {Class} from "./class.entity";
+
+/**
+ * Invitation of a teacher into a class (in order to teach it).
+ */
+@Entity()
+export class TeacherInvitation {
+    @ManyToOne({entity: () => Teacher, primary: true})
+    sender!: Teacher;
+
+    @ManyToOne({entity: () => Teacher, primary: true})
+    receiver!: Teacher;
+
+    @ManyToOne({entity: () => Class, primary: true})
+    class!: Class;
+}