Persist address ids on UserAddress module

Otherwise addresses would not be associated with the user at all
Introduce channel attr to Order model

Could be set when importing orders from other storefronts
Add `ship_total` to api order_attributes
Make api helper attributes class attributes

To make it easier to append more attributes. Other applications /
extensions can append their own stuff without having to override
methods. e.g.

        Spree::Api::ApiHelpers.order_attributes.push :locked_at
Make OrderInventory aware of `track_inventory_levels`

It should not create backordered inventory units when it is set `false`
use the new Spree logo [Fixes #3975]
Add changelog entry for #3946
Replaced Taxon :position refs with :child_index=

Fixes #3390
Fixes #3956

Taxon position unnecessarily fights against the awesome_nested_set
API. By using a_n_s's :move_to_child_with_index method we can more
fully leverage acts_as_nested_set.

Conflicts:
	core/app/models/spree/taxon.rb
	core/spree_core.gemspec

Conflicts:
	backend/app/controllers/spree/admin/taxons_controller.rb