Update ConferenceRequiredMixin

Allow staff users to access the conference in the backoffice even if
it is not public.

src/backoffice/views/mixins.py

@@ -31,10 +31,14 @@ class ConferenceRequiredMixin(PermissionRequiredMixin):
             if conference_id is None:
                 try:
                     self._conference = Conference.objects.filter(is_public=True).first()
+                except Conference.DoesNotExist:
+                    try:
+                        if self.request.user.is_staff:
+                            self._conference = Conference.objects.first()
+                    except Conference.DoesNotExist:
+                        pass
                 if self._conference is not None:
                     self.request.session['conference'] = self._conference.slug
-                except Conference.DoesNotExist:
-                    return None
             else:
                 try:
                     self._conference = Conference.objects.accessible_by_user(self.request.user).get(slug=conference_id)
@@ -74,7 +78,7 @@ class ConferenceRequiredMixin(PermissionRequiredMixin):
 
     def dispatch(self, request, *args, **kwargs):
         if self.require_conference and self.conference is None:
-            return redirect('conference_selection')
+            return redirect('backoffice:conference_selection')
         if not self.has_permission():
             raise PermissionDenied('Insufficient privileges.')
         return super().dispatch(request, *args, **kwargs)