Allow students to cancel their reserved drop-in date

Students can log into the portal and reserve a date for a drop-in class, but there isn't a way for students to cancel that reservation/ "unreserve" the date.

Allowing students to cancel their drop-in reservation would make sure that spots in a class are made available if a person can't make it to their drop-in class. Otherwise, the spot they reserved stays booked unless they tell us they can't come and that's a spot that could go to another dancer.