- Moved the code from Submission.is_late to BaseExercise.is_open_for as it needed to be invoked from BaseExercise.is_submission_allowed.
- Added when keyword argument with default of datetime.now() for many methods that compared times with the current time so that it would be possible to use the same current time when these methods are called paralelly during the request.
- Some code formatting in BaseExercise.is_submission_allowed to follow PEP8.
- Comment refactoring for Submission._set_status