#103 Enable/Disable logins

[Luca Garulli]

It would be very useful a simple common way to enable and disable logins.

We can think to a control panel for the entire application with something like:
- application status {up, down, suspended, mantainance, etc.}
- Messages broadcasting to all connected users (but current)

Statuses can be configured to active/disactive the following options:
- login + message if not
- logging
- ?