Enforce uniqueness of evaluator_submission_assignment

[stepchud]

What/Why

As a Challenge Manager I want to ensure that submissions are only assigned to evaluators once. It should not be possible to create a duplicate assignment record with the same user_id and submission_id.

Acceptance Criteria

• evaluator_submission_assignments have a unique index on user_id/submission_id
  • Model validation adds an error message if a duplicate submission assignment occurs