Fix field AssessmentPart.criterion to point to Criterion table.

Migration 0020 incorrectly altered the field to point to the
CriterionOption table.

TNL-826
TNL-62, TNL-72 and TNL-811
Merge pull request #633 from edx/ammar/tnl72

TNL-72 & TNL-62 & TNL-811
Merge pull request #634 from edx/usman/tnl826-assessment-part-criterion-foreign-key

Fix field AssessmentPart.criterion to point to Criterion table.
Merge branches 'master' and 'muzaffar/get-student-info-tnl836' of github.com:edx/edx-ora2 into muzaffar/get-student-info-tnl836