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/search.py

@@ -99,7 +99,7 @@ class OrderSearch(PaginationMixin, ListView):
         """
         return qs.only(
             'id', 'invoice_address__name_cached', 'invoice_address__name_parts', 'code', 'event', 'email',
-            'datetime', 'total', 'status', 'require_approval'
+            'datetime', 'total', 'status', 'require_approval', 'testmode'
         ).prefetch_related(
             'event', 'event__organizer'
         ).select_related('invoice_address')