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

Conflicts:
	frontend/app/views/spree/checkout/edit.html.erb
[backend] Add missing adjustments/_adjustment partial
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
Move spree/shared/address partial into frontend

That's the only place Spree uses it

Related to #3758
Clean up backend address forms

Related to #3758
With the change of shipping categories seed to Default, product seed needs to match. References #3795

Fixes #3811
Re-instate DefaultTax calculator's ability to calculate for an order

This needs to be supported for legacy reasons. Orders coming from Spree 2.1 and older have the tax adjustments applied on the order as a whole.
Add migrations to migrate old adjustments to new adjustments system

Part of the work in #3567