Transactiontask (#268)

* Introduce TransactionAwareTask

As described in
https://blog.hypertrack.io/2016/10/08/dealing-with-database-transactions-in-django-celery/

* Use TransactionAwareTask instead of countdown

… to prevent race conditions when using a newly created object in a
task.

src/pretix/plugins/banktransfer/views.py

@@ -337,7 +337,7 @@ class ImportView(EventPermissionRequiredMixin, ListView):
             'event': self.request.event.pk,
             'job': job.pk,
             'data': parsed
-        }, countdown=1)
+        })
         return redirect(reverse('plugins:banktransfer:import.job', kwargs={
             'event': self.request.event.slug,
             'organizer': self.request.event.organizer.slug,