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.

Pre-sync
```sql
alter table Orders add voucher_total numeric(11,2);
```

Post-sync
```sql
update Orders set voucher_total = 0 where voucher_total is null;
alter table Orders alter column voucher_total set not null;
```