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/plugins/banktransfer/payment.py

@@ -101,6 +101,11 @@ class BankTransfer(BasePaymentProvider):
     def public_name(self):
         return str(self.settings.get('public_name', as_type=LazyI18nString) or self.verbose_name)
 
+    @property
+    def test_mode_message(self):
+        return _('In test mode, you can just manually mark this order as paid in the backend after it has been '
+                 'created.')
+
     @property
     def settings_form_fields(self):
         d = OrderedDict(