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/base/payment.py

@@ -88,6 +88,18 @@ class BasePaymentProvider:
         """
         return self.settings.get('_enabled', as_type=bool)
 
+    @property
+    def test_mode_message(self) -> str:
+        """
+        If this property is set to a string, this will be displayed when this payment provider is selected
+        while the event is in test mode. You should use it to explain to your user how your plugin behaves,
+        e.g. if it falls back to a test mode automatically as well or if actual payments will be performed.
+
+        If you do not set this (or, return ``None``), pretix will show a default message warning the user
+        that this plugin does not support test mode payments.
+        """
+        return None
+
     def calculate_fee(self, price: Decimal) -> Decimal:
         """
         Calculate the fee for this payment provider which will be added to
@@ -713,6 +725,11 @@ class ManualPayment(BasePaymentProvider):
     identifier = 'manual'
     verbose_name = _('Manual payment')
 
+    @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 is_implicit(self):
         return 'pretix.plugins.manualpayment' not in self.event.plugins