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/order/delete.html

@@ -0,0 +1,31 @@
+{% extends "pretixcontrol/event/base.html" %}
+{% load i18n %}
+{% block title %}
+    {% trans "Delete order" %}
+{% endblock %}
+{% block content %}
+    <h1>
+        {% trans "Delete order" %}
+    </h1>
+    <p>{% blocktrans trimmed %}
+        Do you really want to delete this order? <strong>You really cannot revert this action and we can't either.</strong>
+    {% endblocktrans %}</p>
+
+    <form method="post" href="">
+        {% csrf_token %}
+        <div class="row checkout-button-row">
+            <div class="col-md-4">
+                <a class="btn btn-block btn-default btn-lg"
+                        href="{% url "control:event.order" event=request.event.slug organizer=request.event.organizer.slug code=order.code %}">
+                    {% trans "No, take me back" %}
+                </a>
+            </div>
+            <div class="col-md-4 col-md-offset-4">
+                <button class="btn btn-block btn-danger btn-lg" type="submit">
+                    {% trans "Yes, delete order" %}
+                </button>
+            </div>
+            <div class="clearfix"></div>
+        </div>
+    </form>
+{% endblock %}

src/pretix/control/templates/pretixcontrol/order/index.html

@@ -16,6 +16,9 @@
         {% blocktrans trimmed with code=order.code %}
             Order details: {{ code }}
         {% endblocktrans %}
+        {% if order.testmode %}
+            <span class="label label-warning">{% trans "TEST MODE" %}</span>
+        {% endif %}
         {% include "pretixcontrol/orders/fragment_order_status.html" with order=order class="pull-right" %}
     </h1>
     {% if 'can_change_orders' in request.eventpermset %}
@@ -24,6 +27,13 @@
             {% csrf_token %}
             <div class="btn-toolbar" role="toolbar">
                 <div class="btn-group" role="group">
+                    {% if order.testmode %}
+                        <a href="{% url "control:event.order.delete" event=request.event.slug organizer=request.event.organizer.slug code=order.code %}"
+                                class="btn btn-danger">
+                            <span class="fa fa-trash"></span>
+                            {% trans "Delete" %}
+                        </a>
+                    {% endif %}
                     {% if order.require_approval and order.status == 'n' %}
                         <a href="{% url "control:event.order.approve" event=request.event.slug organizer=request.event.organizer.slug code=order.code %}"
                                 class="btn btn-primary">