Support batch voucher creation

[serial-coder]

To create multiple vouchers with one transaction signing will significantly facilitate the voucher creator

[totiz]

As of Katinrun meeting 0x03 (Security Tech Deep Dive).
We had created 100 Vouchers for estimate 70 pax for participants in the event.

So I think we should add this feature before next event. May be next month ^_^.

PS: It takes around 20 seconds for each Voucher, so takes ~30 mins for 100 vouchers.