[xml/rdf views] handle case where the user hasn't read access to a relation (closes #1984598) The view shouldn't crash in such case. Also refactor xmlrelateditem view and its call on the way.

#!/bin/sh -e
if [ "$1" = "purge" ] ; then
        update-rc.d cubicweb remove >/dev/null
fi
 
#DEBHELPER#
 
exit 0