Update to trainee interviewer scheduling on direct booking links

We've updated how trainees in interviewer pools are scheduled for direct booking links. Previously, if trainees weren't available for scheduling (for example, if they had no availability or had already graduated), candidates wouldn't be able to schedule their interview.

This new update treats Trainees as "optional" when no trainees are left in the interviewer pool (for example, if all trainees have graduated or if none are available for scheduling), allowing the interview to still be scheduled. If the interviewer pool has available trainees and the interview schedule has a trainee slot, a trainee will still be required for scheduling.