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/event.py

@@ -48,7 +48,7 @@ class EventSerializer(I18nAwareModelSerializer):
 
     class Meta:
         model = Event
-        fields = ('name', 'slug', 'live', 'currency', 'date_from',
+        fields = ('name', 'slug', 'live', 'testmode', 'currency', 'date_from',
                   'date_to', 'date_admission', 'is_public', 'presale_start',
                   'presale_end', 'location', 'has_subevents', 'meta_data', 'plugins')