Unpaid fees reminder - group by recipient

[jssmk]

Group all unpaid fee reminders by recipient and payment type, so that reminders become a single email with a list of unpaid fees, instead of individual mail for every missing payment.

If unpaid fees differ from each other by payment type (reference number and bank account number), split into a different reminder mails. If fees differ by fee amount, they could be displayed in same message. List of unpaid fees should include at least payment type, due date and fee amount.

Add total sum and corresponding virtual barcode for listed unpaid fees.