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/services/mail.py

@@ -130,6 +130,8 @@ def mail(email: str, subject: str, template: Union[str, LazyI18nString],
                 body_plain += "\r\n\r\n-- \r\n"
 
             if order:
+                if order.testmode:
+                    subject = "[TESTMODE] " + subject
                 body_plain += _(
                     "You are receiving this email because you placed an order for {event}."
                 ).format(event=event.name)

src/pretix/base/services/orders.py

@@ -533,6 +533,7 @@ def _create_order(event: Event, email: str, positions: List[CartPosition], now_d
             datetime=now_dt,
             locale=locale,
             total=total,
+            testmode=event.testmode,
             meta_info=json.dumps(meta_info or {}),
             require_approval=any(p.item.require_approval for p in positions),
             sales_channel=sales_channel