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/orders/index.html

@@ -114,9 +114,11 @@
                             <strong>
                                 <a
                                         href="{% url "control:event.order" event=request.event.slug organizer=request.event.organizer.slug code=o.code %}">
-                                    {{ o.code }}
-                                </a>
+                                    {{ o.code }}</a>
                             </strong>
+                            {% if o.testmode %}
+                                <span class="label label-warning">{% trans "TEST MODE" %}</span>
+                            {% endif %}
                         </td>
                         <td>
                             {{ o.email|default_if_none:"" }}

src/pretix/control/templates/pretixcontrol/orders/refunds.html

@@ -55,6 +55,9 @@
                                 <a href="{% url "control:event.order" event=request.event.slug organizer=request.event.organizer.slug code=r.order.code %}">
                                     {{ r.order.code }}</a>-R-{{ r.local_id }}
                             </strong>
+                            {% if r.order.testmode %}
+                                <span class="label label-warning">{% trans "TEST MODE" %}</span>
+                            {% endif %}
                         </td>
                         <td>
                             {{ r.payment_provider.verbose_name }}