Don't try to do feedback on sessions without start time

These sessions aren't listed in the session list or schedule, but if
they are linked to externally we would allow them and then crash later
when checking the starttime. So instead of crashing, just disallow the
feedback form on sessions that shouldn't have it.

Author: mhagander

postgresqleu/confreg/views.py

@@ -1134,7 +1134,7 @@ def feedback(request, confname):
 def feedback_session(request, confname, sessionid):
     # Room for optimization: don't get these as separate steps
     conference = get_conference_or_404(confname)
-    session = get_object_or_404(ConferenceSession, pk=sessionid, conference=conference, status=1)
+    session = get_object_or_404(ConferenceSession, pk=sessionid, conference=conference, status=1, can_feedback=True, starttime__isnull=False)
 
     if not conference.feedbackopen:
         # Allow conference testers to override