[refactoring] introduce add_etype_button function to properly create button to add a new entity
also properly use cw_create_url to generate the URL to add the entity
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0