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Story Description
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When the deadline for a submission has passed, the instructor can assign
submissions to specific Graders so that they can start marking the
submissions. It should be possible to "reuse" certain mapping between Graders
and submissions, in order to reduce the work required by the instructor.

Tests
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* Test if "reuse" of submission-Grader mappings work as expected
* Check that each submission is assigned to *one* Grader only
(no double-mappings)
* Test if mappings show up appropriately for Grader users