Be able to indicate per customer/invoice whether the g-account is active

Now this is either on or off and depending on the invoicing setting "G account active" and whether there are rates attached to the invoice over which an amount should be sent to the g account.

However, this should only be active for a few customers within the customer base and for these customers sometimes for only a few invoices. It would be nice if, just like with VAT reverse charge, there is the possibility to switch the g account on/off per customer and to overrule this per invoice.

Think about whether we want to set this setting active by default or not when adding a new customer.