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/search/orders.html

@@ -59,9 +59,11 @@
                     <td>
                         <strong>
                             <a href="{% url "control:event.order" event=o.event.slug organizer=o.event.organizer.slug code=o.code %}">
-                                {{ o.event.slug|upper }}-{{ o.code }}
-                            </a>
+                                {{ o.event.slug|upper }}-{{ o.code }}</a>
                         </strong>
+                        {% if o.testmode %}
+                            <span class="label label-warning">{% trans "TEST MODE" %}</span>
+                        {% endif %}
                     </td>
                     <td>{{ o.event.name }}</td>
                     <td>