Add a multi-column index on spree_reviews.approved + spree_reviews.pr…

…oduct_id

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).

db/migrate/20171004122538_index_product_id_and_approved.rb

@@ -0,0 +1,5 @@
+class IndexProductIdAndApproved < ActiveRecord::Migration[5.1]
+  def change
+    add_index :spree_reviews, [:approved, :product_id]    
+  end
+end