An extreme situation in previous state could happend, that daily backup
starts just a minute after hourly backup starts.

By spanning hourly backups across 35 minutes and start other from 45 and
later, we give 10 minutes at least to hourly backup to finish.

We also add 5 minutes before every whole hour to let system take care of
removed backups (snapshots).

And uninstallation task in case of mail delivery is not enabled

Conditioning virtual users configuratoin by mail_delivery_enabled flag
was not actualy correct

exim is not needed anymore, postfix does its work

It is suitable to use as system e-mail sender as well

Allow to specify extra allowed e-mails and domains for virtual users

- New simp_le version needs `-f account_reg.json` plugin enabled
- Update staging to v2 API URL in comment