Add a multi-column index on spree_reviews.approved + spree_reviews.product_id

[nzaillian]

Index these columns as they are used for scoping records in the stock gem controllers (and likely used in any custom controllers users are likely to write as well).

Context: in our application we have 10s of ks of reviews across ~60 products. We've discovered that these columns are unindexed and there is at this size a big performance consequence (like ~60ms+ in a pretty well-provisioned Postgres db). It seems the compound index is the one that's most performant. If you think it makes sense to additionally add a single-column index on product_id I can do this as well (I think the multi-column supports the query on only approved = ? so I don't think there needs to be an additional index on approved, but I don't think it supports a query on product_id only - but you're not likely to be making such a query in a frontend I don't think...).