Add validation of shipping_category_id to Product. Fixes #3188

Not sure if we want to include this on 2-0-stable or not as it's an api change.

The issues this resolves are marked for 2.0.4 milestone though:
https://github.com/spree/spree/issues/3188
https://github.com/spree/spree/issues/3347

Also need to update guides, which I started although not sure if other pages need updates:
https://github.com/spree/spree-guides/pull/147