Implement automated account deletion (fixes #64)

Also see docker-create-account!4 (merged).

TODO:

• 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