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