Add a simple test mode (#1181)

- [x] Provide data model and configuration toggle
- [x] Allow to delete individual test orders
- [x] Add tests
- [x] Add a prominent warning message to the backend if test mode orders exist (even though test mode is off), as this leads to wrong statistics
- [x] Decide if and how to generate invoices for test orders as invoice numbers cannot be repeated or should not have gaps.
- [x] Decide if and how we expose test orders through the API, since our difference pull mechanism relies on the fact that orders cannot be deleted.
- [x] Decide if and how we want to couple test modes of payment providers?
- [ ] pretix.eu: Ignore test orders for billing
- [ ] Adjust payment providers: Mollie, bitpay, cash, fakepayment, sepadebit

![download](https://user-images.githubusercontent.com/64280/53009081-fe420d80-343a-11e9-8361-b8511c988598.png)

src/pretix/control/views/orders.py

@@ -11,7 +11,9 @@ from django.conf import settings
 from django.contrib import messages
 from django.core.files import File
 from django.db import transaction
-from django.db.models import Count, IntegerField, OuterRef, Subquery
+from django.db.models import (
+    Count, IntegerField, OuterRef, ProtectedError, Subquery,
+)
 from django.http import (
     FileResponse, Http404, HttpResponseNotAllowed, JsonResponse,
 )
@@ -398,6 +400,35 @@ class OrderApprove(OrderView):
         })
 
 
+class OrderDelete(OrderView):
+    permission = 'can_change_orders'
+
+    def post(self, *args, **kwargs):
+        if self.order.testmode:
+            try:
+                with transaction.atomic():
+                    self.order.gracefully_delete(user=self.request.user)
+                messages.success(self.request, _('The order has been deleted.'))
+                return redirect(reverse('control:event.orders', kwargs={
+                    'event': self.request.event.slug,
+                    'organizer': self.request.organizer.slug,
+                }))
+            except ProtectedError:
+                messages.error(self.request, _('The order could not be deleted as some constraints (e.g. data created '
+                                               'by plug-ins) do not allow it.'))
+                return self.get(self.request, *self.args, **self.kwargs)
+
+        return redirect(self.get_order_url())
+
+    def get(self, *args, **kwargs):
+        if not self.order.testmode:
+            messages.error(self.request, _('Only orders created in test mode can be deleted.'))
+            return redirect(self.get_order_url())
+        return render(self.request, 'pretixcontrol/order/delete.html', {
+            'order': self.order,
+        })
+
+
 class OrderDeny(OrderView):
     permission = 'can_change_orders'