Change RSVP to Invited for non-RSVP events

[BrianRamsay]

For events that are not expecting RSVP, we need to change our language to 'Invited' students, if people are added ahead of time. Wherever we display those RSVP students, both before and after the event, should reflect this change.

The backend referencing and storing these students does not need to change, unless it helps for clarity.

[BrianRamsay]

Fix volunteer RSVP language in non-RSVP event