Fix time zone activation leaking into subsequent requests in require_admin_access()

The timezone of an admin user was only activated, but never deactivated,
so it leaks into following requests of the current thread.

For example, this affected tests executed AFTER
wagtail.admin.tests.test_account_management (see #9628).

This commit changes the timezone activation to use the override()
context manager from django.utils.timezone, which calls deactivate()
when the context manager is closed.

This is similar to how we use the override() from
django.utils.translation for locale activation in the same decorator.

wagtail/admin/auth.py

@@ -2,12 +2,13 @@ import types
 from functools import wraps
 
 import l18n
+from django.conf import settings
 from django.contrib.auth import get_user_model
 from django.core.exceptions import PermissionDenied
 from django.db.models import Q
 from django.shortcuts import redirect
 from django.urls import reverse
-from django.utils.timezone import activate as activate_tz
+from django.utils.timezone import override as override_tz
 from django.utils.translation import gettext as _
 from django.utils.translation import override
 
@@ -175,8 +176,9 @@ def require_admin_access(view_func):
                     )
                     l18n.set_language(preferred_language)
                     time_zone = user.wagtail_userprofile.get_current_time_zone()
-                    activate_tz(time_zone)
-                with LogContext(user=user):
+                else:
+                    time_zone = settings.TIME_ZONE
+                with override_tz(time_zone), LogContext(user=user):
                     if preferred_language:
                         with override(preferred_language):
                             response = view_func(request, *args, **kwargs)