Bug 19166: Add the ability to add adjustments to an invoice

Bug 19166: Add the ability to add adjustments to an invoice

This patchset adds the ability to add adjustments to an invoice, one can
provide a reason, an adjustment amount, select a budget, and choose
whether to encumber the funds before the invoice is closed or not

To test:

1 - Create a new invoice with or without a shipping cost
2 - Note there are no existing adjustments
3 - Add an adjustment
4 - Submit the form withno changes, nothing happens
5 - Update the adjustment you created, ensure changes are saved but no
extra adjustment created
6 - Add another invoice prodiving only reason or amount (you can have 0
        value adjustments)
7 - Verify the adjustment total at bottom is correct
8 - Recieve some orders
9 - Verify totals are correct

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>