Eager load producer properties to avoid N+1
Eager load producer properties to avoid N+1

This totally removes the following N+1 query from the GET /shops request

```sql
SELECT "spree_properties".* FROM "spree_properties"
  INNER JOIN "producer_properties"
    ON "spree_properties"."id" = "producer_properties"."property_id"
  WHERE "producer_properties"."producer_id" = 17
  ORDER BY producer_properties.position;
```

The producer properties of the corresponding enterprises are now loaded
in a single query fired from the controller.

Matt-Yorkley/openfoodnetwork,lin-d-hop/openfoodnetwork,mkllnk/openfoodnetwork,openfoodfoundation/openfoodnetwork,lin-d-hop/openfoodnetwork,openfoodfoundation/openfoodnetwork,Matt-Yorkley/openfoodnetwork,lin-d-hop/openfoodnetwork,openfoodfoundation/openfoodnetwork,Matt-Yorkley/openfoodnetwork,Matt-Yorkley/openfoodnetwork,lin-d-hop/openfoodnetwork,mkllnk/openfoodnetwork,mkllnk/openfoodnetwork,mkllnk/openfoodnetwork,openfoodfoundation/openfoodnetwork