Truncate order items when itemization is enabled and an order has more than 30

When itemization of orders is enabled, we need to truncate long line items lists since Accept.js won't accept API requests with more than 30 items. We should count the number of items we've added, and for the 30th create a truncated item whose quantity is the number of remaining order items, amount is the combined amount of those items, and whose title is "other items". The description should be, "This order exceeded the 30 item limit for transactions in Authorize.Net. Refer to the order on your site for the full item list."

I actually don't think it's required to identify teach line item as taxable or not, and I don't know that we're doing that for our line items right now anyways. Choose the simplest approach here for that consolidated line item that preserves required data integrity.