ECA model for declined sessions

Problem/Motivation

It is possible (albeit inadvisable) for camp organizers to schedule sessions that are accepted but not yet confirmed. If they do and then a session is declined, we could have an ECA model to handle it.

Proposed resolution

Have an ECA mode that will act if a session moves to the Declined state:
If the room has been set, unset it
If the time slot has been set, unset it
Email the site (or maybe other emails?) to notify that the session has been declined