mail_attach_existing_attachment_account

Extend invoice sending to let users pick existing invoice attachments when emailing from the accounting send wizard.

Key Features

account.move.send.wizard: Add a checkbox list of the invoice's existing attachments for selection in the send wizard
account.move.send.wizard: Enable the option to attach existing invoice attachments by default when opening the send wizard
account.move.send.wizard: Include user-selected invoice attachments in the extra attachments sent with the email