Add lineitemkey to UI

We integrate inbound and outbound orders and pass along a line item number with the lineitemkey field. This works great for the most part. However, if we need to manually add a line from the order, we are not able to enter a line number. This causes problems with the import of the information back into our ERP system because there is no line number to match. Bringing the existing lineitemkey field to the UI with the ability to edit will alleviate several of the issues that we are facing. This change will not require a new field, but provide the ability to use an existing field that we rely heavily on for our integrations.