Categorization is dead. Long live the categorizer!

Summary: Switch out the categorizer that's shown so content creators will start using the new one for new items. We'll leave the old one around as long as we still have items that use it. (Which won't be too long thanks to an impending backfill)

Test Plan: none

Reviewers: jack, joel

Reviewed By: joel

Differential Revision: http://phabricator.khanacademy.org/D9465