webui: custom facet groups in a facet
    
    Facets use to inherit facet groups from entity. There was no option to define
    cross-entity facet groups for different facets which belong one entity.
    
    In other words it was not possible to have 'user search' and 'stage user search'
    tab in one facet group.
    
    Reviewed-By: David Kupka <dkupka@redhat.com>
    Reviewed-By: Thierry Bordaz <tbordaz@redhat.com>