[web/css] move jquery.treeview.css override to a separate file
Patched embedded code copies are a maintenance disaster (even more so
than plain embedded code copies). Let's not do that.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/