This fixes the error UK's is experiencing:
```
PG::AmbiguousColumn: ERROR: column reference "state" is ambiguous LINE
1: SELECT DISTINCT spree_orders.*, CASE WHEN state IN ('cancele...
^ : SELECT DISTINCT spree_orders.*, CASE WHEN state IN ('canceled',
'returned') THEN payment_total WHEN state IS NOT NULL THEN payment_total
- total ELSE 0 END AS balance_value, spree_orders.* FROM "spree_orders"
INNER JOIN "spree_shipments"
```
See
https://app.bugsnag.com/yaycode/openfoodnetwork-uk/errors/6058c45989d37300079e8312?event_id=6058ccd30075af73bcb20000&i=sk&m=nw.
We rely now on the exhaustive list of states an order can be in after
checkout. What made this all a bit more messy is that I made up the
"checkout" order state, likely mixing it from the payment model states.
This simplifies things quite a bit and gives meaningful names to things.
