Store tax_category on LineItem model

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). It would be faster to not have to do that, and so storing tax_category on the line item would aid in this.