Tornado handlers for logging into the notebook.
The basic tornado login handler
authenticates with a hashed password from the configuration.
Called by handlers.get_current_user for identifying the current user.
See tornado.web.RequestHandler.get_current_user for details.
Whether this LoginHandler is needed - and therefore whether the login page should be displayed.
Return the hashed password from the tornado settings.
If there is no configured password, an empty string will be returned.
Check the notebook application’s security.
Show messages, or abort if necessary, based on the security configuration.