Fix field AssessmentPart.criterion to point to Criterion table.

Migration 0020 incorrectly altered field AssessmentPart.criterion to point to the CriterionOption table
instead of the Criterion table even though the model field pointed to the Criterion model. Maybe it was hand written?

I have also changed the model serialisations in the earlier migrations so that they are consistent with the state of the database.

https://openedx.atlassian.net/browse/TNL-826