Rename "static" dir into "header"
Rename static module to header
rename static to header (temporary: pdf will return to "static" module)
Renaming directories from static to header
Keep static/ in proxy for retrocompatibily issues (existing URLs).

It mght diseappear in a future release.
Update release notes (redaction may be improved - feel free to do it)
Merge remote-tracking branch 'origin/master' into renamestatic

Conflicts:
	geonetwork
Do not provide an nonexistent PDF URL by default - better empty

And modules that use it should (they do) manage the case of an
empty URL.

See also following issue for GeoNetwork submodule:
https://github.com/georchestra/geonetwork/issues/13
Empty default pdf URL in geonetwork

See https://github.com/georchestra/geonetwork/issues/13 for two
remaining mentions to default pdf URL.
Merge pull request #521 from georchestra/renamestatic

Rename static to header