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/templates/pretixcontrol/checkin/index.html

@@ -89,6 +89,9 @@
                                 {% if e.order.status == "n" %}
                                     <span class="label label-warning">{% trans "unpaid" %}</span>
                                 {% endif %}
+                                {% if e.order.testmode %}
+                                    <span class="label label-warning">{% trans "TEST MODE" %}</span>
+                                {% endif %}
                             </td>
                             <td>{{ e.item }}{% if e.variation %} – {{ e.variation }}{% endif %}</td>
                             <td>{{ e.order.email }}</td>