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/event.py

@@ -242,6 +242,8 @@ class Event(EventMixin, LoggedModel):
 
     :param organizer: The organizer this event belongs to
     :type organizer: Organizer
+    :param testmode: This event is in test mode
+    :type testmode: bool
     :param name: This event's full title
     :type name: str
     :param slug: A short, alphanumeric, all-lowercase name for use in URLs. The slug has to
@@ -271,6 +273,7 @@ class Event(EventMixin, LoggedModel):
     settings_namespace = 'event'
     CURRENCY_CHOICES = [(c.alpha_3, c.alpha_3 + " - " + c.name) for c in settings.CURRENCIES]
     organizer = models.ForeignKey(Organizer, related_name="events", on_delete=models.PROTECT)
+    testmode = models.BooleanField(default=False)
     name = I18nCharField(
         max_length=200,
         verbose_name=_("Event name"),