Allow vouchers to apply a one time fee during signup and a fixed trial date

This update adds a new "Scheduled trial" voucher discount type so a subscription voucher can charge a one-time intro fee and delay full billing to a fixed calendar date, e.g. "249 kr to sign up now, then the normal 99 kr/month plan starts billing on Jan 1st". The one-time fee is optional - a "Scheduled trial" voucher can also be configured to just delay billing to a fixed date with no charge at all.