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/urls.py

@@ -222,6 +222,8 @@ urlpatterns = [
             name='event.order.approve'),
         url(r'^orders/(?P<code>[0-9A-Z]+)/deny$', orders.OrderDeny.as_view(),
             name='event.order.deny'),
+        url(r'^orders/(?P<code>[0-9A-Z]+)/delete$', orders.OrderDelete.as_view(),
+            name='event.order.delete'),
         url(r'^orders/(?P<code>[0-9A-Z]+)/info', orders.OrderModifyInformation.as_view(),
             name='event.order.info'),
         url(r'^orders/(?P<code>[0-9A-Z]+)/sendmail$', orders.OrderSendMail.as_view(),