Cancelling classes email

This emails students with SSIs in a class when the last section of the class is cancelled. It also creates a new template for this email. I tested that it emails students with a valid SSI that are not otherwise being emailed.
This template also duplicates text, and means that the email body that admins get is no longer correct.  It might just be easier to write a single email that can apply to both, just with a different email code depending on whether it's a class or a section.