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/context.py

@@ -52,6 +52,15 @@ def contextprocessor(request):
 
         ctx['has_domain'] = request.event.organizer.domains.exists()
 
+        if not request.event.testmode:
+            complain_testmode_orders = request.event.cache.get('complain_testmode_orders')
+            if complain_testmode_orders is None:
+                complain_testmode_orders = request.event.orders.filter(testmode=True).exists()
+                request.event.cache.set('complain_testmode_orders', complain_testmode_orders, 30)
+            ctx['complain_testmode_orders'] = complain_testmode_orders
+        else:
+            ctx['complain_testmode_orders'] = False
+
         if not request.event.live and ctx['has_domain']:
             child_sess = request.session.get('child_session_{}'.format(request.event.pk))
             s = SessionStore()