add counter cache from variants to stock items

This adds a counter cache for stock items.

While running bullet and deleting some option types, it recommended me to setup a counter cache for stock items. I decided to do so. This was the result of that effort.
It appeared to be what everyone was doing on the internet.

We probably shouldn't change the `created_at`, but I have no qualms against changing the `updated_at`. Should I remove it?