Added "purchase-pre-order" substitution so that copying orders for
next week is not tied to time invoices are sent out (though this can still happen too). Fixed some stock tracking bugs when adding purchases, also wasn't checking the quantity of composite items being added to purchases. Removing purchases can now be configured to only remove purchases where quota hasn't been met. Stock adjustments are now stored as the delta rather than the new total so that reconcilliation can be done more easily.
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