[Avalara] Don't break out order items

[brent-hoover]

Currently the order items (not cart items) are "exploded" out, adding a new line item for every quantity (e.g. 30 X Item = 30 line items with a quantity of 1).

We want to change the behavior so that order items are kept as an aggregate until they are operated upon, e.g. split-shipped/returned etc. Then they should be broken out.

This change should be made in a backwards-compatible way so that people with existing orders don't experience any issues.

[brent-hoover]

I have this working but I don't know this is going to work with people who have existing orders. Trying to figure out a migration strategy.

[brent-hoover]

Closed via #2119