Retrieve the top-level story for a comment

[telotortium]

I think that each comment inserted into the database should include a column onstory that contains the ID of the story on which the comment was made. This is exactly equivalent to the link after "on:" at the top of an HN comment page (example). We could do this either by directly retrieving the HTML page and using Beautiful Soup to find that link, or alternatively recurse up the tree in the Firebase API using the parent field (probably using functools.lru_cache in case a person has commented a bunch of times on the same story).