[server] install custom sql scripts before creating tables for the schema
A cube might need something like 'CREATE EXTENSION postgis' before
creating tables for its entities, to add new final types.
usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py