fix assessment list to make distinct by assessment

[shapiromatron]

Fix bug where /assessment/all/ duplicated assessments based on the number of people assigned to various roles. The previous implementation made duplicative assessments since it was generating a row for each assessment-user combination; to dedup using the distinct() disabled our ability to have a custom sort ordering.

The new approach breaks adds an Exists annotation to the assessment so we prevent the duplicates; query performance is actually faster despite the extra annotations.

[dannypeterson]

neat use of djangos built in classes and interesting fix! approved