Store tax category on line item

Fixes #3481, real proper like.

> Currently, to calculate a tax rate for a line item, we reach through the associated variant to the product and then the product's tax category.  [See here for what I mean](https://github.com/spree/spree/blob/e99c7b1d0649ccaf13c74760d59f092807bd8979/core/app/models/spree/calculator/default_tax.rb#L34-L35) for what I mean. It would be faster to not have to do that, and so storing tax_category on the line item would aid in this.