Server Messages unpublish

* changes unpublish_on to optional value
* simplifies fetching of server message news

konova/views/home.py

@@ -38,13 +38,7 @@ def home_view(request: HttpRequest):
     user_teams = user.shared_teams
 
     # Fetch the four newest active and published ServerMessages
-    msgs = ServerMessage.objects.filter(
-        is_active=True,
-        publish_on__lte=now,
-        unpublish_on__gte=now,
-    ).order_by(
-        "-publish_on"
-    )[:3]
+    msgs = ServerMessage.get_current_news()[:3]
 
     # First fetch all valid objects (undeleted, only newest versions)
     interventions = Intervention.objects.filter(