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/paypal/payment.py

@@ -36,6 +36,18 @@ class Paypal(BasePaymentProvider):
         super().__init__(event)
         self.settings = SettingsSandbox('payment', 'paypal', event)
 
+    @property
+    def test_mode_message(self):
+        if self.settings.connect_client_id and not self.settings.secret:
+            # in OAuth mode, sandbox mode needs to be set global
+            is_sandbox = self.settings.connect_endpoint == 'sandbox'
+        else:
+            is_sandbox = self.settings.get('endpoint') == 'sandbox'
+        if is_sandbox:
+            return _('The PayPal sandbox is being used, you can test without actually sending money but you will need a '
+                     'PayPal sandbox user to log in.')
+        return None
+
     @property
     def settings_form_fields(self):
         if self.settings.connect_client_id and not self.settings.secret: