rename static to header (temporary: will return to a "static" module")

Note also #10. If #10 s merged into https://github.com/georchestra/geonetwork/tree/georchestra-13.12, please also merge https://github.com/georchestra/geonetwork/tree/renamestatic into https://github.com/georchestra/geonetwork/tree/georchestra-13.12.