webui: fail nicely if cookies are disabled
    
    Reworks also sessionStorage test because disablement of cookies might be connected
    with sessionStorage and localStorage. E.g. Chrome raises exception when *Storage
    is accessed with "Block sites from setting any data" settings set in
    "Content Settings/Cookies" section.
    
    https://fedorahosted.org/freeipa/ticket/4338
    
    Reviewed-By: Tomas Babej <tbabej@redhat.com>