Enforce access control based on scheduling rules

The aim here is to move away from the clunky distinction between scheduling rules and using the 'scheduler' module to publish and unpublish nodes on a schedule. The two use cases are closely coupled, and we want to avoid the risk of information leak that could occur by having an unscheduled (but published) slide be accessible, e.g. via the normal Drupal node routes.

So, we now aim to ensure we systematically just use the Drupal access mechanisms to determine whether or not an unscheduled slide ought to be accessible.