Enable waitlist setting changes on a per-class basis, not global

We have a student who registered for a class that has openings because she is waiting on charter school funding. Because we have the season setting that prohibits parents from adding a student to a class when it has an opening, rather than wait in line for available openings after previous waitlisted students, this means no other students can register for the class even though it has openings.

We need a way to override that setting so we can maintain the waitlisted student but allow other students to register for classes with openings.