[16.0][ADD] website_sale_digital_improved

[hbrunn]

This module extends the functionality of website_sale_digital:

• allow downloading digital products without portal account
• allow attaching digital products to the invoice mail

Usage

Access to digital products for customers without a portal account works
out of the box. Note that the links to digital products are designed to
be valid for an hour, so in case you receive complaints from your
customers that they don't work, ask them to refresh the page showing the
sale order.

To have Odoo mail digital products along with the invoice, enable
Attach to invoice mail on a digital attachment of a product. This
only works as expected if you're using an instant payment method, and
have enabled Automatic Invoice in the website settings.