webui: login screen - improved button switching
    
    - added cancel button to reset password view of login screen
    - re-implemented buttons hiding mechanism
    - switching between 'Reset Password' and 'Reset Password and Login' according to presence of value in OTP field
    
    https://fedorahosted.org/freeipa/ticket/4470
    
    Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>