New administration page to configure email settings

This page should allow to define and test the properties of the email notification channel :

• smtp host (empty by default, which means that Email channel disabled)
• smtp port (25 by default)
• use secure connection - "No" and "SSL" ("No" by default)
• smtp username (empty by default)
• smtp password (empty by default)
• from address ("noreply@nowhere" by default)
• email prefix ("[SONAR]" by default)