E-invoice support

[vohmar]

related to #329

while in the ticket #329 auto invoicing the focus is on automatic invoice generation and delivery on set parameters.

But starting from the 1st of July all Estonian government institution accept only e-invoices. While auto invoicing concentrates on delivering the invoice to the bank so registrar can configure automatic payments in gov case we need to send the e-invoices to the e-invoice center. For this user has to have an option to request an invoice to be sent to e-invoice center. The option should enable user to set the e-invoice as a default form and delivery method for invoice so whenever new invoice is created it is instantly sent as e-invoice to user defined destination (bank/e-invoice center)

[vohmar]

Let's leave the automated sending out for now - after creating an prepayment invoice user must have an option to select the e-invoice delivery (bank and eak) similarly to send by email option.