Add three modules helping on automatically invoicing customers.

The base module `account_invoice_base_invoicing_mode` does not actually
do anything but adds a selection field on partner to allow assigning an
invoicing mode to a customer. And a checkbox to choose regrouping
invoices.

The two other modules add specific invoicing mode.

The module `account_invoice_mode_monthly` creates monthly invoices for
customer on a specific day (configuration is in Accounting Settings)

The module `account_invoice_mode_at_shipping` creates invoices on the
shipping of the goods.

Those modules use queue_job to generate and validate invoices.

Add few changes from reviews.
Use sudo to post the invoice.

Split invoice creation on invoice partner settings that decides if sales order
atre being grouped in invoice or not.
The previous implementation would crash.

…one_invoice_per_order' to res.partner's _commercial_fields()