Fixes issue with StudentExercise creation due to Follow-up Question validations  

This PR should correct the issues seen with StudentExercise creation.  Follow-up Question validations were using the SE's LearningCondition, but the join used the id of the as-of-yet unsaved SE, causing nil to be returned.  This has been corrected.

In addition, the validations for missing Follow-up Answers has been changed to ensure the Student cannot lock in his free-response answers.