Require scheduling for all slides

Since Drupal sets the end date to the value of the start date if no end date is provided by the user, let's require that all slides be scheduled with both a start and end date.

Some clients may have slides with no scheduling, so I've left the filtering criteria on the views alone.

See the corresponding task: http://projects.sitenow.uiowa.edu/projects/digital-signage/tasks/40