update Scratchpad and ScratchpadRevision to be ndb (to be or ndb, that is the question)

Summary:
joel converted these models to ndb.  when models get converted we need to change this config.  good
to keep in mind for the future.

Test Plan: i edited the prod file on the analytics machine, and gae_download.py resumed downloading data

Reviewers: james

Reviewed By: james

CC: chris

Differential Revision: http://phabricator.khanacademy.org/D1071