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.

Conflicts:
	core/app/helpers/spree/products_helper.rb
	core/app/models/spree/line_item.rb
	frontend/app/views/spree/orders/_line_item.html.erb