Handle user session expiration

• The user should be informed if his session has expired. Ideally, the application should show a dialog where would be a button that would start the authentication.
• The expiration can be detected by subscribing to a particular event of the UsersManager in the AuthService.
• To force the login, it should be enough to refresh the page. The application will detect that the user is not logged in and will redirect him to the login page.