Formalize a process for removing Exams/Reports

[ssciolla]

This application is still relatively new, but at some point, we will need to be able to remove specific Exam or Report instances. Deleted Exam instances will result in related Submissions being deleted (CASCADE), but Reports being deleted will simply set the FK to null. (Exams and associated Reports should probably be removed together to ensure integrity.)

In a pinch, we could issue custom SQL to delete a Report and/or an Exam, but it would be better if these changes were captured somehow in a log and fully covered by unit tests. This kind of change management could be integrated with work on #54.