borgbackup: Support for hetzner storage box

Hetzner Storage-Boxes are having native support for borg-backup. They cannot be used by backupninja because

• they require port 23 instead of 22 for ssh-connections using borg (!31 (merged))
• they don't provide a shell on port 23, thats why the read/write-checks are failing and ssh-copy-id is not working. Therefore we need a workaround for ssh-copy-id by downloading, modifying and uploading ~/.ssh/authorized_keys via sftp (see also: https://github.com/openssh/openssh-portable/blob/35ff70a/contrib/ssh-copy-id#L299 and https://bugzilla.mindrot.org/show_bug.cgi?id=3201) (!36)