Add migration to associate line items and iu directly

Part of #3991
Update Shipment `set_up_inventory` to work with line_items

Part of #3991
Package `add` receives a line item as first argument

Part of #3991

So that it can set the line item when creating inventory units. Variant
would still be fetched from the line item
Set completed_at after line items are created in order_factory

Part of #3991

As it represents a more real use case. Having a empty completed order
would cause inventory issues when addin line items. e.g. It would try to
create inventory units for line items not yet persisted on the database
(read activerecord callback hell)
Fix specs related to package api changes

Had to modify the order factory which caused a bunch of other specs to
fail. Hopefully now it has a more reasonable approach which is first
create line items list then shipments and then update! itself.

Part of #3991
Remove Stock::RemainingPacker (not referenced anywhere)

Part of #3991

Had some broken specs there and decided to remove it rather then fix the
specs. It's not used anywhere and it's not clear why it exists so we
might be better off without it
Verify inventory after line_item is saved

We need the line item id when creating inventory units.

Part of #3991
Use find_each in AddLineItemIdToSpreeInventoryUnits migration to not load all the shipments at once

Part of #3991
Add missing let block for variant to order_inventory_spec
Correct check for variant-level inventory tracking in OrderInventory
Remote erb inline comment from backend/payment_method/form

Motivation is to avoid parse errors when trying to deface that file

See #3913
use a better name for the method that restarts the checkout flow,also make sure we use the checkout_steps.first instead of hardcoding it to "address".

use expect syntax

use the correct syntax

Conflicts:
	core/app/models/spree/order.rb

Fixes #3995
(backend) in order details, line items states could not be translated

Fixes #4009
[backend] verifying if product picker returns any product

Fixes #4008
do not call move_to_child_with_index when the taxon is new, since this
is not supported by acts_as_nested_set.

Fixes #3999
Show log entries in the admin backend

Working towards a solution for #3978
[backend] Fix issue where database cleaner was not truncating tables
[backend] Use Shipment#display_cost rather than #cost
Bump kaminari to 0.15.0
Go with whatever money gem activemerchant depends on