add back button in edit pages (#96)

closes #89

Reviewed-on: #96

frontend/allowance-planner-v2/src/app/pages/edit-task/edit-task.page.ts

@@ -23,7 +23,8 @@ export class EditTaskPage implements OnInit {
     private formBuilder: FormBuilder,
     private taskService: TaskService,
     private userService: UserService,
-    private router: Router
+    private router: Router,
+    private location: Location
   ) {
     this.id = this.route.snapshot.params['id'];
     this.isAddMode = !this.id;
@@ -77,4 +78,8 @@ export class EditTaskPage implements OnInit {
     this.taskService.deleteTask(this.id);
     this.router.navigate(['/tabs/tasks']);
   }
+
+  navigateBack() {
+    this.location.back();
+  }
 }