remove broken variant "deleted" scope

this "deleted" scope doesn't remove the default "deleted_at: nil" condition so it just produces a query like this:

```sql
SELECT "spree_variants".*
FROM "spree_variants"
WHERE "spree_variants"."deleted_at" IS NULL
AND   "spree_variants"."deleted_at" IS NOT NULL
```

instead you have to use the "only_deleted" scope from the acts_as_paranoid gem
see https://github.com/goncalossilva/acts_as_paranoid/blob/ae0fe74/README.markdown#filtering

example: https://github.com/spree/spree/pull/4562

Fixes #4601
Support existing credit card feature on api#checkout

about #4598
provide consistent sort order for promotions

make pagination sane for promotions.
especially true for postgres which seems to do more to keep its promise that "If sorting is not chosen, the rows will be returned in an unspecified order".
since there are no indexes on this table we just use the primary key with most recent first.

Fixes #4596
Fix thank you message on order completion

Thank you message was no more displayed since it was using a wrong
params to test when order was just completed.
This solve the issue by using flash[:commerce_tracking] param
to test when to display the message. An helper method has been
added to aviod code duplication.
Use a more relavant naming and avoid using useless
'nothing special' string as content of the flash message.

Fixes #4586
Refactor Shipments API & JS.

Conflicts:
	backend/app/assets/javascripts/spree/backend/shipments.js.erb
Remove deprecated orders/shipments api.

Fixes #4523
Make sure line_item is assigned in specs

As of dcbd5de we expect line_items to always be associated with an
inventory_unit. Ensure we are assigning that in specs.
Importer::Order should connect Invntory to Items

The Spree::Core::Importer::Order allows importing inventory units via
the API.

Since InventoryUnits should now be associated with LineItems, this ties
the unit to the first line_item matching the imported variant.
Remove order association from inventory_units

There is no need to store the order_id on an inventory_unit, as they are
tied to both a line_item in an order and a shipment in an order.
Instead, we can delegate order to the shipment.

This should hopefully allow some future simplification as they are more
normalized.