Added support for default e-mail suffix for notifications (and relevant unit tests).
This applies to users that don't have their e-mail field set: if sonar.properties contains  "email.address_auto_suffix=@foo.bar" then the notification will be sent to "username@foo.bar".
Please note suffix should include "@", this allows broader support, for example ".group@domain".

This can come in handy when the user authentication system does not provide the e-mail ; quite often users are reachable to username@somedomain.
Furthermore, in such a case, currently even if users manually type in their e-mail, it is cleared upon next login (see SONAR-4034).