Provide the ability to create registration states that don't use a slot

It would be very useful to have a registration state that doesn't use a slot. We have a course where people need to be vetted before they can be confirmed on the course. So, we don't them to be taking up a slot while we are waiting for them to be vetted.

This would also be useful for the 'cancelled' state. I would expect that setting a registration to 'cancelled' would free up a slot, but it doesn't. You have to delete the registration to free up a slot, which loses all of the registrants data.