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
Include jquery.validate localization messages when the locale is not :en

Fixes #3794
Add a test for #3494
Allow users to delete country via backend UI

Fixes #3807
Allow users to create new countries via backend UI

Fixes #3807
[backend] Remove test that depends on browser to be in focus

See comment thread: https://github.com/radar/spree/commit/6dcddf961589bd0162e09c5ce94b24acd0ba39cb#commitcomment-4218181
With the change of shipping categories seed to Default, product seed needs to match. References #3795

Fixes #3811