Eager load taggings related to variant overrides

This removes an N+1 with taggings but doesn't solve the one with tags.
Using `includes(taggings: :base_tags)` based on
47da5036de/lib/acts_as_taggable_on/taggable.rb (L83-L84)
wasn't enough to solve it and I got to stop here. This is scope-creeping
too much.

app/controllers/admin/variant_overrides_controller.rb

@@ -75,6 +75,7 @@ module Admin
 
     def collection
       @variant_overrides = VariantOverride.
+        includes(:taggings).
         joins(variant: :product).
         preload(variant: :product).
         for_hubs(params[:hub_id] || @hubs)