voucher functionality

Allow to assign vouchers for specialist visit to the subject.

Each voucher should have:

• serial number with checksum - I was thinking about 4-5 digits for the serial number + 2 digits for checksum
• issue date
• expiry date (for now it's issue date+3 months)
• use date (automatically filled when status changed to used)
• voucher type
• status: new, used, expired, deleted (only for internal data consistency)
• feedback from Voucher Partner: free text
• voucher partner