[request] Ensure base_url() always has a trailing '/' (closes #3955093)
Just as an example, cubicweb.ajax.js is clearly written under the
assumption that BASE_URL/baseuri() - which simply contains req.base_url() - has a
trailing /.
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)