Deleting Exam removes associations from Videos

* Deleting an exam will first look up all the videos which have a
  foreignkey to it, and sets all their .exam properties to None. Then it
  goes ahead with the deletion.
  (closes #2043)
Merge pull request #2127 from Stanford-Online/jrbl-2043-delete-exam-deletes-relationships-too

Deleting Exam removes associations from Videos