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/api/views/order.py

@@ -16,7 +16,7 @@ from rest_framework.exceptions import (
     APIException, NotFound, PermissionDenied, ValidationError,
 )
 from rest_framework.filters import OrderingFilter
-from rest_framework.mixins import CreateModelMixin
+from rest_framework.mixins import CreateModelMixin, DestroyModelMixin
 from rest_framework.response import Response
 
 from pretix.api.models import OAuthAccessToken
@@ -51,10 +51,10 @@ class OrderFilter(FilterSet):
 
     class Meta:
         model = Order
-        fields = ['code', 'status', 'email', 'locale', 'require_approval']
+        fields = ['code', 'status', 'email', 'locale', 'testmode', 'require_approval']
 
 
-class OrderViewSet(CreateModelMixin, viewsets.ReadOnlyModelViewSet):
+class OrderViewSet(DestroyModelMixin, CreateModelMixin, viewsets.ReadOnlyModelViewSet):
     serializer_class = OrderSerializer
     queryset = Order.objects.none()
     filter_backends = (DjangoFilterBackend, OrderingFilter)
@@ -378,6 +378,13 @@ class OrderViewSet(CreateModelMixin, viewsets.ReadOnlyModelViewSet):
     def perform_create(self, serializer):
         serializer.save()
 
+    def perform_destroy(self, instance):
+        if not instance.testmode:
+            raise PermissionDenied('Only test mode orders can be deleted.')
+
+        with transaction.atomic():
+            self.get_object().gracefully_delete(user=self.request.user if self.request.user.is_authenticated else None, auth=self.request.auth)
+
 
 class OrderPositionFilter(FilterSet):
     order = django_filters.CharFilter(field_name='order', lookup_expr='code__iexact')