Add next re-certification date to PDF certificates

When re-certifiaction of PDF certificates is enabled, the next re-recertification date can be calculated and added to the certificate as a "valid-until" element of the certificate.

The following variables are added to the PDF generation process:

dateNextRecertification : short date format
dateNextRecertificationLong : long date format
dateNextRecertificationRaw : raw date format (only when using the Phantom-Worker