Don't cascade user deletion into deletion of exams etc

Thom Wiggersrequested to merge
dont_cascade_user_deletion_into_education into master

Previous behaviour

Steps to reproduce:

  1. try to delete userid 1
  2. See that it tries to nuke a lot of exams

New behaviour

Steps to validate that it works:

  1. nuke user 1 without extensive fallout.

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