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/serializers/order.py

@@ -231,7 +231,7 @@ class OrderSerializer(I18nAwareModelSerializer):
 
     class Meta:
         model = Order
-        fields = ('code', 'status', 'secret', 'email', 'locale', 'datetime', 'expires', 'payment_date',
+        fields = ('code', 'status', 'testmode', 'secret', 'email', 'locale', 'datetime', 'expires', 'payment_date',
                   'payment_provider', 'fees', 'total', 'comment', 'invoice_address', 'positions', 'downloads',
                   'checkin_attention', 'last_modified', 'payments', 'refunds', 'require_approval', 'sales_channel')
 
@@ -413,7 +413,7 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
 
     class Meta:
         model = Order
-        fields = ('code', 'status', 'email', 'locale', 'payment_provider', 'fees', 'comment', 'sales_channel',
+        fields = ('code', 'status', 'testmode', 'email', 'locale', 'payment_provider', 'fees', 'comment', 'sales_channel',
                   'invoice_address', 'positions', 'checkin_attention', 'payment_info', 'consume_carts')
 
     def validate_payment_provider(self, pp):