Preserve metadata for deleted submissions #222
Description
This can help pinpoint students that abuse the submission system in combination with a very debugging-friendly grader to extract test data case by case.
Only the uploaded file can be sensitive student data, so we can and should preserve the submission metadata. Doing this will also require a clear way to mark deleted submissions in the UI, and might require a(nother) redo of the logic that determines which submission gets considered for grading.