Allow actual Write-off Process for Invoices

[wyldbill]

Currently there is no way to write-off an invoice. This is a common practice in many business domains. Work-arounds don't generally provide the correct functionality.

A write-off should:

1. Retain all the line-item information of the invoice.
2. Make the effective balance of the invoice $0
3. Close the invoice - it can effectively be considered PAID, though an additional status would be nice.

A write-off should not:

1. Result in a positive or negative entry in Transactions. 
2. Contribute to totals of any status (PAID, OVERDUE, etc.)

Note that Orders allow for COMPING the entire order. This is effectively the same desired functionality, as it leaves the order intact, marks it as complete, but does not indicate any revenue associated with the Order. (I'm making assumptions, because we don't use Orders)

Workarounds that don't work:

• Create a 100% discount. This is possible, and would essentially be perfect, but results in a $0 total, which disallows the saving of the invoice.
• Create a negative cost line-item. This is not allowed (that's what discounts are for) and would have the same problem as a discount.
• Mark the invoice as PAID (Other). This is suggested elsewhere. This results in a transaction with a positive value, which is incorrect. A written-off invoice is NOT Paid, so this actually causes and accounting error.
• Archive the invoice and ignore it. Archiving is great, but the outstanding amount from the invoice still contributes to the "Overdue" total at the top of the page (which is really confusing, as you may not be able to see any overdue invoices by default, but this value may still be non-zero)
• Canceling the Invoice. This is the closest option but doesn't allow stating WHY the invoice was cancelled and the invoice total doesn't equal zero.