Enhance adding volunteers through RSVP Modal

As of now, the RSVP modal on Manage Volunteers page will automatically handle adding too many people and will waitlist all volunteers over the RSVP limit. Some possible enhacncement changes:
- Add an indicator in the RSVP modal of how many people can RSVP
- Visually group those who will be RSVPd and those who will be waitlisted so they can be sure of their actions. Right now it is kind counterintuitive as the first people you RSVP get waitlisted if you go over the limit, instead of the last.