Allow editing a cancellation reason after the cancellation has been processed

Members often email asking to cancel, and staff need to process the cancellation promptly - but members don't always give their reason for leaving until later. This means staff sometimes have to select a reason just to complete the cancellation, which reduces the accuracy of cancellation reporting and makes it harder to spot genuine trends or issues worth addressing.

It would be really useful to be able to update a cancellation reason retrospectively, once the member has provided the correct information.