A teacher can set 'Available from' and 'Due date' for an assignment

1. Login as a teacher, update an 'Upload a single file' assignment and set an 'Available from' date.
2. Login as a student and check that the 'Available from' date is displayed in the assignment and that before this date there is no upload file option.
3. Login as a teacher, update the 'Upload a single file' assignment and set a due date. Set 'Prevent late submissions' to Yes.
4. Login as a student and check that the due date is displayed in the assignment and also in the course calendar.
5. Check that after the due date there is no longer an upload file option.