Automatic logoff of non-admin users and redirection to login page.

Non-admin users are automatically logged out when the system is under maintenance, and redirected
to the login screen. In the login screen a banner is shown on the top displaying the maintenance
message set by the administrator. This way, admin users can log in even under maintenance.
refactoring
Merge branch 'jgonzalezdr-master'
require redmine 2.4