webui: login screen widget
    
    Reimplementation of unauthorized dialog into separate widget. It uses RCUE
    design.
    
    New features compared to unauthorized dialog:
    
    - reflects auth methods from `auth` module
    - validation summary
    - differentiates Kerberos auth failure with session expiration
    - Caps Lock warning
    - form based method doesn't allow password only submission
    
    https://fedorahosted.org/freeipa/ticket/4017
    https://fedorahosted.org/freeipa/ticket/3903
    
    Reviewed-By: Adam Misnyovszki <amisnyov@redhat.com>