allow any authenticated user to access the schedule preview API endpoint

see: https://github.com/ansible/tower/issues/1939

awx/api/views.py

@@ -720,6 +720,7 @@ class SchedulePreview(GenericAPIView):
     model = Schedule
     view_name = _('Schedule Recurrence Rule Preview')
     serializer_class = SchedulePreviewSerializer
+    permission_classes = (IsAuthenticated,)
 
     def post(self, request):
         serializer = self.get_serializer(data=request.data)