#983 deactivation warning

Resolves #983.
I'm not sure, since we need to display the confirmation if there are overbooked dates and only proceed with the deactivation code if the user confirms. There are two independent conditions where you would proceed with deactivation (either there are overbooked days and the user confirms or there are no overbooked days) and I'm not sure if the logic be any cleaner. I'll think over it before merging.