webui: prefer search facet's deleter dialog
    
    Deleter dialog in search facet is now chosen in order as follows:
    - facet's, defined as spec, e.g.:
      deleter_dialog: { $factory: IPA.user.deleter_dialog }
    - entity's, the same but it entity spec
    - default, which is IPA.search_deleter_dialog
    
    Previous didn't allow to override entity dialog with facet one and
    also definition by spec was not allowed.
    
    Reviewed-By: David Kupka <dkupka@redhat.com>
    Reviewed-By: Thierry Bordaz <tbordaz@redhat.com>