Time machine mode [Z#23129725] (#3961)

Allows organizers to test their shop as if it were a different date and time.

Implemented using a time_machine_now() function which is used instead of regular now(), which can overlay the real date time with a value from a ContextVar, assigned from a session value in EventMiddleware.

For more information, see doc/development/implementation/timemachine.rst

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>
Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/control/templates/pretixcontrol/event/live.html

@@ -46,7 +46,7 @@
                             {% endfor %}
                         </ul>
                     </div>
-                    <div class="test-right">
+                    <div class="text-right">
                         <button type="submit" class="btn btn-primary btn-lg btn-save" disabled>
                             {% trans "Go live" %}
                         </button>
@@ -82,10 +82,10 @@
                     <p>
                         {% trans "Your shop is currently in test mode. All orders are not persistent and can be deleted at any point." %}
                     </p>
-                    <div class="form-inline">
-                        <label class="checkbox">
+                    <div class="checkbox">
+                        <label>
                             <input type="checkbox" name="delete" value="yes" />
-                            {% trans "Permanently delete all orders created in test mode" %}
+                            <b>{% trans "Permanently delete all orders created in test mode" %}</b>
                         </label>
                     </div>
                     <div class="text-right">

src/pretix/control/templates/pretixcontrol/event/transfer_session.html

@@ -0,0 +1,9 @@
+{% load eventurl %}
+{% load static %}
+<form action="{% eventurl request.event "presale:event.auth" %}{% if request.GET.next %}?next={{ request.GET.next }}{% endif %}" method="post">
+    <input type="hidden" value="{{ new_session }}" name="session">
+    <button type="submit">
+        Continue
+    </button>
+</form>
+<script src="{% static "pretixcontrol/js/send_form.js" %}"></script>