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/multidomain/urlreverse.py

@@ -180,7 +180,7 @@ def build_absolute_uri(obj, urlname, kwargs=None):
     """
     Works similar to ``eventreverse`` but always returns an absolute URL.
 
-    :param obj: An ``Event`` or ``Organizer`` object
+    :param obj: An ``Event`` or ``Organizer`` object, or ``False`` to generate main domain URLs
     :param name: The name of the URL route
     :type name: str
     :param kwargs: A dictionary of additional keyword arguments that should be used. You do not
@@ -188,7 +188,10 @@ def build_absolute_uri(obj, urlname, kwargs=None):
         needed.
     :returns: An absolute URL (including scheme and host) as a string
     """
-    reversedurl = eventreverse(obj, urlname, kwargs)
+    if obj is False:
+        reversedurl = mainreverse(urlname, kwargs)
+    else:
+        reversedurl = eventreverse(obj, urlname, kwargs)
     if '://' in reversedurl:
         return reversedurl
     return urljoin(settings.SITE_URL, reversedurl)