Extract get_turtle_message().

This seems like kind of a silly function to extract
to topic.py, but it will theoretically help us sweep
"subject" if we change the DB.

It had test coverage.

zerver/lib/topic.py

@@ -227,3 +227,12 @@ def get_topic_history_for_web_public_stream(recipient: Recipient) -> List[Dict[s
     cursor.close()
 
     return generate_topic_history_from_db_rows(rows)
+
+def get_turtle_message(message_ids: List[int]) -> Message:
+    # This is used for onboarding, and it's only extracted
+    # here to make subject -> topic sweeping easier.
+    turtle_message = Message.objects.get(  # nolint
+        id__in=message_ids,
+        subject='topic demonstration',
+        content__icontains='cute/turtle.png')
+    return turtle_message