hide "print vouchers" link when there is no vouchers

suggestion for a minor improvement. On the subject edit page, the print vouchers link could be hidden or disabled when no vouchers are present.

created branch 218-hide-print-vouchers-link-when-there-is-no-vouchers

mentioned in merge request !144 (merged)

changed milestone to %0.12.0

closed via merge request !144 (merged)