The time that add-ons add to an appointment is now worked out correctly when a customer books.
- Per-person add-ons: everyone receives the add-on during the same appointment, so its time is added once, not once for every person in the booking.
- Add-ons for the whole booking: if the same add-on is chosen more than once, its time is now added for each one chosen.
- Add-ons that are no longer offered: if an add-on has been archived, belongs to a different location or is no longer offered for that service, the booking is stopped with a message that the add-on is no longer available, instead of going ahead with it.
