backoffice: on invalid activation code a 500 was displayed

Caused by referencing an unbound variable in the logger.warning() call
-.-

src/backoffice/views/auth.py

@@ -84,6 +84,7 @@ class SignupDoneView(ConferenceMixin, TemplateView):
 def signup_activate(request, uidb64, channel_id, token):
     try:
         uid = force_text(urlsafe_base64_decode(uidb64))
+        user, channel = None  # ensure variables exists if any of the next two fail, otherwise the logger call fails
         user = PlatformUser.objects.get(pk=uid)
         channel = user.communication_channels.get(id=channel_id)
     except Exception as err: