Pass through contact block from service to letter

Whatever a user has entered for their service’s contact block should
appear in the right place in the file we give to DVLA.

The work to output in the right fields in the DVLA file has already been
done. We just weren’t passing it through. This commit passes it through.

app/celery/tasks.py

@@ -274,6 +274,7 @@ def build_dvla_file(self, job_id):
                     # This unique id is a 7 digits requested by DVLA, not known
                     # if this number needs to be sequential.
                     numeric_id=random.randint(1, int('9' * 7)),
+                    contact_block=notification.service.letter_contact_block,
                 ))
                 for notification in dao_get_all_notifications_for_job(job_id)
             )