When a user change the volunteers type to involve driving they should be prompted to select a driving time slot. As it stands they are just signed up for the 10:30 slot regardless.
They should have the option to change the date the volunteer is signed up for. If the volunteer is a special event volunteer and they get asked to be moved to next week and the special event is not running they should just be signed up for the next regular event.
This expands on #88