base: drop-down menu fix

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>

Conflicts:
	invenio/base/static/less/base.less
	invenio/base/templates/header_base.html
base: drop-down menu fix

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>