add @atomic decorator to view method

This allows AppointmentWorkflowStepCompletion and Study updates to
participate in the same database transaction.

I decided against moving the step completion code into the StudyService
as logically they are unrelated, and they belong to separate apps.
Doing the former update in the view is also more consistent with what
the other views are already doing.

Author: MatMoore

manage_breast_screening/mammograms/views/appointment_views.py

@@ -248,6 +248,7 @@ def get_context_data(self, **kwargs):
         )
         return context
 
+    @transaction.atomic
     def form_valid(self, form):
         form.save(
             StudyService(appointment=self.appointment, current_user=self.request.user)