Remove N+1 on variants and products

We get from an initial INNER JOIN with variants and products to fetch
the variant overrides + N queries like:

```sql
SELECT "spree_variants".* FROM "spree_variants" WHERE
"spree_variants"."deleted_at" IS NULL AND "spree_variants"."id" = $1
LIMIT 1  [["id", 1545]]

SELECT "spree_products".* FROM "spree_products" WHERE
"spree_products"."id" = $1 LIMIT 1  [["id", 604]]
```

to the same initial INNER JOIN + just 2 queries like:

```sql
SELECT "spree_variants".* FROM "spree_variants" WHERE
"spree_variants"."deleted_at" IS NULL AND "spree_variants"."id" IN
(1551, 1554)

SELECT "spree_products".* FROM "spree_products" WHERE
"spree_products"."deleted_at" IS NULL AND "spree_products"."id" IN (606,
607)
```

app/controllers/admin/variant_overrides_controller.rb

@@ -76,6 +76,7 @@ module Admin
     def collection
       @variant_overrides = VariantOverride.
         joins(variant: :product).
+        preload(variant: :product).
         for_hubs(params[:hub_id] || @hubs)
     end