[web] Add datadir-url configuration option (closes #5204550) Makes it possible to use a different host for static data instead of ${base_url}/data

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl