Refactor students#show.
Validate grades on assignment, task, and submission IDs.
Add unique index to grades table.
Test unique index on grade model.
Fix typos.
Fix 'student' refs in grade tests.
Fix Grade test assertion.
Cleanup and refactoring.
Cleanup Students#show and memoize course_data fields.
Merge pull request #459 from bellotastudios/unique_grades

[Reviewed] Unique grades for student / assignment.