Topic metadata generator: skip deleted topics

Not really a big deal, but deleted topics are getting sent back through
this consumer when the clean_private_data script erases their data,
since that changes the markdown and puts them into the topic.edited
queue. There shouldn't be any reason to process deleted topics and
re-add "blank" metadata (0 word count, no excerpt), so we can just skip
them.

tildes/consumers/topic_metadata_generator.py

@@ -33,6 +33,9 @@ class TopicMetadataGenerator(PgsqlQueueConsumer):
             self.db_session.query(Topic).filter_by(topic_id=msg.body["topic_id"]).one()
         )
 
+        if topic.is_deleted:
+            return
+
         if topic.is_text_type:
             new_metadata = self._generate_text_metadata(topic)
         elif topic.is_link_type: