Allow manual total when doing a refund with item adjustment

[travisirby]

For refunds of invoices that have line items with associated tax lines, when choosing to do item adjustments as part of the refund the total amount to refund is a read only field that is calculated by the items chosen to adjust.

This is problematic for the use-case where I want to refund the full invoice total, but I only want to adjust the non tax line items. This is because the tax line items will be automatically adjusted when their associated line items are adjusted. If the tax line items are selected for adjustment in the refund page the adjustment will be more than the total of the invoice creating a positive account balance. If I only select the non tax line items for adjustment the refund will not be for the total amount of the invoice.

I have confirmed that the behavior in kill bill using direct kill bill api call works as expected if I set the amount to refund to the total of the invoice and only include the non tax line items in the adjustments parameter. So this appears to be only a kaui limitation