Fix /changes breaks due to multiple rows returned

In the previous days of Serveradmin we allowed to restore objects with
the same id. This can lead to an object being in the history as deleted
multiple times.

serveradmin/serverdb/views.py

@@ -72,9 +72,12 @@ def changes(request):
     # or from the change entry where the object was deleted.
     server_hostname = Server.objects.filter(
         server_id=OuterRef('object_id')).values('hostname')
+    # Workaround: In previous versions of Serveradmin restoring objects with
+    # the same id was possible, so we can end up with objects being deleted
+    # multiple times. We only take the latest into account.
     change_hostname = Change.objects.filter(
         object_id=OuterRef('object_id'),
-        change_type=Change.Type.DELETE).order_by('-id').values('change_json')
+        change_type=Change.Type.DELETE).values('change_json').order_by('-id')[:1]
     commits = commits.prefetch_related(Prefetch(
         'change_set',
         queryset=Change.objects.all().annotate(hostname=Coalesce(