Add new trigger: Post is scheduled

[andergmartins]

This trigger fires when a post is saved as scheduled. If the date changes, there should have an option to trigger it again. This can be useful for creating notifications in advance, before the post is published.

Use case – we have a post set to expire (i.e. become a draft) 14 days after publication. We want the author to be notified 7 days in advance of that change and 24 hours in advance of that change.