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/models/invoices.py

@@ -150,6 +150,8 @@ class Invoice(models.Model):
         if not self.prefix:
             self.prefix = self.event.settings.invoice_numbers_prefix or (self.event.slug.upper() + '-')
         if not self.invoice_no:
+            if self.order.testmode:
+                self.prefix += 'TEST-'
             for i in range(10):
                 if self.event.settings.get('invoice_numbers_consecutive'):
                     self.invoice_no = self._get_numeric_invoice_number()