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

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

src/pretix/presale/urls.py

@@ -176,6 +176,8 @@ event_patterns = [
     re_path(r'^(?P<subevent>\d+)/widget/product_list$', pretix.presale.views.widget.WidgetAPIProductList.as_view(),
             name='event.widget.productlist'),
 
+    re_path(r'timemachine/$', pretix.presale.views.event.EventTimeMachine.as_view(), name='event.timemachine'),
+
     # Account management is done on org level, but we at least need a logout
     re_path(r'^account/logout$', pretix.presale.views.customer.LogoutView.as_view(), name='organizer.customer.logout'),
 ]