ItemAdjustments refactored

Remove callbacks from ItemAdjustments

It is unclear what the benefit of the callbacks were here. They're not
demarking a change of state or something complex. They're just
for inserting into the middle of a fairly short sequence of actions.
With it broken into smaller methods, it's now fairly easy to override
one of the methods and call super in a sub-class.

Renamed ItemAdjustments as Adjustable::AdjustmentsUpdater

This is a method object, not a model, so its more consistant to place
it in a subfolder. It also doesn't operate an "Item" model, but on any
of the models that fall under the "Adjustable" association.

Propagate ItemAdjustment name change

Add note to changelog

Closes #5739
Promotion actions refactored

Make adjustments on LineItem not Order

Closes #5738
remove merge conflict that was still there.
the Spree::ItemAdjustments model is no longer here,
commented out the shared examples in the specs for now.
fix typo in promotion policy select
get the currency on the package from the order.

fixes #5746
closes #5753
Fix for failure of one of multiple payments.

This corrects an issue which would leave the user on the confirm page
when one of multiple payments failed. This would allow the user to
proceed through the confirm step with an invalid payment, not billing
them for the full amount required.

An example would be with the store credit payment method, where they are
able to pay for part of the order with store credit. Example, the
customer has $20 store credit available and is trying to purchase a $100
item.

Currently, if attempting to charge $80 to the credit card fails (due to
the card being declined), they $80 credit card payment will be flagged
invalid, leaving only the $20 payment. The user would remain on the
confirm page, which allows them to check out paying $20 on their store
credit. The user will beleieve that everything worked correctly, however
their $20 store credit has been consumed, but the order is still in a
balance due state.

This is fixed by only allowing an order to remain on the confirm page if
they have sufficient payments to pay for the entire order.

Closes #5754
speed up country creation

speed up state creation

fixes #5730
allow no variant products to be eligilbe in the exchange for

prevent n+1 query by using is_master

fixes #5749
fixes #5745