Fix field AssessmentPart.criterion to point to Criterion table.

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

TNL-826
More accurate validation for date & time in studio editor.

TNL-58
Added email address to AUTHORS.
Using ISO_8601 for date formatting instead of string.

TNL-58
Comment refactoring.

TNL-58
Adding more readable comments to catch block.

TNL-58
Merge pull request #638 from edx/muzaffar/tnl58-date-validations

More accurate validation for date & time in studio editor.
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.
Merging master into this branch