Give ShippingCategory correct name so sample shipments have shipping_rates

Fixes #3795
Support .name and .description on line items

This allows line_items to override and control what should be used for
description and name sanely.

The extra helper isn't ideal, but we do maintain the old interface for
extensions.

Fixes #3734
Do not allow one product to be assigned the same taxon more than once

Fixes #3494
Correct shipping category name for sample data
Include jquery.validate localization messages when the locale is not :en

Fixes #3794
Add a test for #3494