A teacher can restrict the time period in which students can view and/or add entries to a database activity

1. Login as a teacher and set available from and to dates for a database activity.
2. Login as a student and check that the activity is only available between the available from and to dates. Outside these dates a message explaining when the activity will be available or when it ceased to be available will be displayed.
3. Login as a teacher again, remove the available from and to dates and set viewable from and to dates.
4. Login as a student and check that between the viewable from and to dates the 'Add entry' tab is not displayed.