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/base/models/memberships.py

@@ -23,7 +23,6 @@ from django.db import models
 from django.db.models import Count, OuterRef, Subquery, Value
 from django.db.models.functions import Coalesce
 from django.utils.formats import date_format
-from django.utils.timezone import now
 from django.utils.translation import gettext_lazy as _
 from django_scopes import ScopedManager, scopes_disabled
 from i18nfield.fields import I18nCharField
@@ -31,6 +30,7 @@ from i18nfield.fields import I18nCharField
 from pretix.base.models import Customer
 from pretix.base.models.base import LoggedModel
 from pretix.base.models.organizer import Organizer
+from pretix.base.timemachine import time_machine_now
 from pretix.helpers.names import build_name
 
 
@@ -165,13 +165,13 @@ class Membership(models.Model):
 
     def is_valid(self, ev=None, ticket_valid_from=None, valid_from_not_chosen=False):
         if valid_from_not_chosen:
-            return not self.canceled and self.date_end >= now()
+            return not self.canceled and self.date_end >= time_machine_now()
         elif ticket_valid_from:
             dt = ticket_valid_from
         elif ev:
             dt = ev.date_from
         else:
-            dt = now()
+            dt = time_machine_now()
 
         return not self.canceled and dt >= self.date_start and dt <= self.date_end