refactor: use item sales uom on new orders

[batonac]

I'm not sure what all history might go into this, but it seems not hard to argue that we should be using the sales uom per item on new order items.

If this is not a safe assumption for all, then we could build a few options in the settings.

[batonac]

@agritheory this is ready for another review, maybe merge 😃

[batonac]

Thanks!