Implement automated account deletion (fixes #64)
Also see docker-create-account!4 (merged).
- create an smtp account so that confirmation emails can be sent (or use an existing account like drive@ or welcome@)
- handle confirmation links correctly
- delete nextcloud account (reports failure for no reason)
- delete postfixadmin account
- make sure that confirmation emails are delivered
- validate the code to ensure that an attacker cant delete arbitary accounts