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

This test requires an assignment with file submissions enabled.

1. Login as a teacher, update the assignment and set an 'Allow submissions from' date.
2. Login as a student and check that the date is displayed in the assignment and that before this date there is no file submission option.
3. Login as a teacher, update the assignment and set a due date and a cut-off date.
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 cut-off date there is no longer a file submission option.