Add a configurable message when canceling a registration and a different message when canceling waitlist registration

Problem/Motivation

When the user clicks to the Delete/Cancel their registration in the email they see this message:

"You are cancelling your registration for for . Once you do this, there may no longer be any spaces left for this event and you may not be able to register again."

This message can be confusing when you are canceling the Waitlist registration.

Suggestion:

• Add 2 config fields to /admin/structure/events/registrant/settings, one for the Registrant Deletion - Waitlist and another for Registrant Deletion - Confirmed spot
• Update RegistrantDeleteForm::getDescription() to return the fields from config according to waitlist value.