[web] Add missing import of _DATA_DIR in webctl Followup on d84bc85f7f70 which introduced usage of this name in _datadirs method of GenStaticDataDirMixIn but resulted into a NameError at runtime and makes migrations crash.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)
embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js