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/control/logdisplay.py

@@ -172,6 +172,7 @@ def pretixcontrol_logentry_display(sender: Event, logentry: LogEntry, **kwargs):
         'pretix.event.order.paid': _('The order has been marked as paid.'),
         'pretix.event.order.refunded': _('The order has been refunded.'),
         'pretix.event.order.canceled': _('The order has been canceled.'),
+        'pretix.event.order.deleted': _('The test mode order {code} has been deleted.'),
         'pretix.event.order.placed': _('The order has been created.'),
         'pretix.event.order.placed.require_approval': _('The order requires approval before it can continue to be processed.'),
         'pretix.event.order.approved': _('The order has been approved.'),
@@ -268,6 +269,8 @@ def pretixcontrol_logentry_display(sender: Event, logentry: LogEntry, **kwargs):
         'pretix.event.plugins.disabled': _('A plugin has been disabled.'),
         'pretix.event.live.activated': _('The shop has been taken live.'),
         'pretix.event.live.deactivated': _('The shop has been taken offline.'),
+        'pretix.event.testmode.activated': _('The shop has been taken into test mode.'),
+        'pretix.event.testmode.deactivated': _('The test mode has been disabled.'),
         'pretix.event.added': _('The event has been created.'),
         'pretix.event.changed': _('The event settings have been changed.'),
         'pretix.event.question.option.added': _('An answer option has been added to the question.'),