Allow superusers to open any story in builder

By default, the list endpoints for stories, assets and datasets filter the
lists to only published items for unauthenticated users and to only a
user's items for authenticated users.  This prevents superusers from
opening another user's story in the builder, making it harder to debug
problems.  This commit allows superusers to see an unfiltered list of items
via API endpoints by removing all queryset filters in the various
``Resource`` subclassses.

Additional testing is needed before *editing* stories by other users
(including superusers) is supported, but this should at least allow us to
investigate JavaScript errors thrown by the builder.

Addresses #657