avoid custom model methods in data migration

The data migration for product ratings does not work correctly
with South 8 (and earlier) due to reliance on methods that are
not included in the frozen model. This behavior was introduced
in #773, which was a step in the right direction, but which fails
when run against a non-empty product catalogue.

This migration mimics the logic of Product.calculate_rating, adding
the frozen models from the reviews app and friends, and avoiding
any custom methods or joins. It also uses a hard-coded value in
place of ProductReview.APPROVED, for better or worse.
Merge pull request #1336 from flashnotes/issue/remove_custom_methods_from_data_migration

avoid custom model methods in data migration