Save orders.voucher_total correctly.

This column was added back when vouchers were moved up to store. No new databases have been created since, so we haven't run into this yet, but as the column is not null, it needs to be set by default. Do that.

To push this live on other sites, the following sql will have to be run against their dbs, or orders will start failing. The timing of this has to be when the new packages go live, or the new not null requirement will also break things.

```sql
alter table Orders add voucher_total numeric(11,2);
update Orders set voucher_total = 0;
alter table Orders add voucher_total set not null;
```