webui: standalone facet
    
    `facet.Facet` is a new base class for facets. It doesn't have any dependencies
    on entities so it's usable for general purpose facets, e.g., future API browser,
    load facet or login facet.
    
    https://fedorahosted.org/freeipa/ticket/3903
    
    Reviewed-By: Adam Misnyovszki <amisnyov@redhat.com>