Notify providers that appointments will only be persisted after they get confirmed

[adewes]

We no longer include appointments in the cloud backup data of the provider and they will not be persisted until the provider has been verified by a mediator. Hence if providers signs up and create appointments before they are verified (which we tell them they can do) and then log out the appointments will all be gone.

We should notify providers of this behavior or disable appointment creation before verification.