Only show necessary timeframes on AK wall

Hides hours no upcoming AK is in. For technical reasons, only one visible area can be displayed for all days, hence hours for one day might be visible even though the are not needed (but just for another day). For improved visibility, cutoff will be with one hour offset. This implements #206 (closed)

AKDashboard/tests.py

-import pytz
+import zoneinfo
 from django.apps import apps
 from django.test import TestCase, override_settings
 from django.urls import reverse
@@ -22,7 +22,7 @@ class DashboardTests(TestCase):
         cls.event = Event.objects.create(
             name="Dashboard Test Event",
             slug="dashboardtest",
-            timezone=pytz.utc,
+            timezone=zoneinfo.ZoneInfo("Europe/Berlin"),
             start=now(),
             end=now(),
             active=True,