[frontend] don't let dev instance notify all users
    
    I am babyproofing the script for notifying about outdated chroots
    and adding one more safety lock. I am really worried, that sooner
    or later someone will send the notification from copr-fe-dev to
    all Copr users. In this commit I am forbidding this scenario.
    This script should be executed on dev only for testing purposes
    and therefore only subset of users (e.g. only yourself, or Copr team)
    should be even notified.