Fix for issue 1437. Had some issues with github on friday so here I am a...

Pull request for the fix for issue #1437.

The problem was that the warning icon (that indicates if a submission to an assignment is late) is appearing even when the user submitted on time. The root cause was the system is looking at the last commit of the users entire repo, not the specific assignment itself.

The fix makes the submissions page gather last submission dates from the correct assignment repo directory and is now able to show the handy warning icon only when the submission is late for that assignment.

Tested both on single users and groups. Had user1 submit assignment on time for A1 and the submit to A2 after the due date of A1. Then had user2 submit late assignment to A1. Verified that user1's submission record on A1 has no warning icon while user2's late submission does.

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