FR: remind the user not set postfix IP address in the settings

We've noticed that many users have not setup the postfix IP address in their settings, which is a dependency for enabling most of the value of lightmeter, nameny:

• RBL checks
• Peer network.

Some of the approaches we can use:

• document this need in the README
• reminder the user with notifications until it's done
• make the postfix IP address a hard requirement, preventing controlcenter to run without it and making it mandatory in the registration page or settings.
  • for new users, having it in the registration page is easier, but it won't work for existing setups
  • for existing setups, we should remind via notifications.

For notifications, we could use the existing status messages subsystem, as well as e-mail notifications.