Improved warning for manual waitlist moves

When an admin or staff member manually moves a customer off of a waitlist, the system now provides a better warning before confirmation. If the class or series is already full, the system will alert the user that this action will result in overbooking. This improvement is particularly beneficial for cases where a Series reservation is moved off the waitlist, as the system makes it clear that the move impacts the entire series, not just an individual class. The instance they are viewing may not be full, but the series as a whole may be at capacity (because individual reservations may be allowed within the series).