Added prepayment functionality for invoice report

There is a new setup in the wood setup (sales) to automatically archive the document on prepayment posting. 

If this is active, the sales order will be archived before posting an prepayment invoice/credit memo. 

By default the prepayment invoice lines are of type g/l account. Therefore it is not possible to print all information the same way as on other documents. With this change, the archived version can be used to print the item information on the documents.

If this setup is active, the layout is based on the archived version of the sales order so that prepayment invoice looks like this: