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/base.html

@@ -235,6 +235,14 @@
                             </ul>
                         </div>
                         <ul class="nav" id="side-menu">
+                            {% if request.event and request.event.testmode %}
+                                <li class="testmode">
+                                    <a href="{% url "control:event.live" event=request.event.slug organizer=request.organizer.slug %}">
+                                        <i class="fa fa-warning fa-fw"></i>
+                                        {% trans "TEST MODE" %}
+                                    </a>
+                                </li>
+                            {% endif %}
                             {% block nav %}
                                 {% for nav in nav_items %}
                                     <li>
@@ -317,6 +325,20 @@
                             </div>
                         {% endfor %}
                     {% endif %}
+                    {% if complain_testmode_orders %}
+                        <div class="alert alert-warning">
+                            {% blocktrans trimmed %}
+                                Your event contains <strong>test mode orders</strong> even though <strong>test mode has been disabled</strong>.
+                                You should delete those orders to make sure they do not show up in your reports and statistics and block people from
+                                actually buying tickets.
+                            {% endblocktrans %}
+                            <strong>
+                                <a href="{% url "control:event.orders" event=request.event.slug organizer=request.organizer.slug %}?status=testmode">
+                                    {% trans "Show all test mode orders" %}
+                                </a>
+                            </strong>
+                        </div>
+                    {% endif %}
                     {% if warning_update_check_active %}
                         <div class="alert alert-info">
                             <a href="{% url "control:global.update" %}">