blacklist system logins for aliases and logins

We blacklist based on three things:
* blacklist in APP_CONFIG[:handle_blacklist]
* emails in RFC 2142
* usernames in /etc/passwd

The latter two can be allowed by explicitly whitelisting them in APP_CONFIG[:handle_whitelist].

We stick to blocking names that have been configured as both blacklisted and whitelisted - better be save than sorry.
Merge pull request #102 from azul/feature/3602-email-blacklist

blacklist system logins for aliases and logins