[FIX] website_sale_digital: let salesman download digital via portal

Portal shows all Sale Orders available for current user. For example, salesman
can see his sales. If such a user can download digital files via product form in
backend, it makes sense to let user download them via SO page on portal.
However, it wasn't the case because /my/download requires product be purchased
by current user [1]. Fix it by checking read access first.

STEPS

* in backend create SO with digital product (customer must be different from
current user)
* create invoice and register a payment
* navigate to portal  (without using customer's token),
* open SO, click download on digital product

[1]: https://github.com/odoo/odoo/blob/1a24477fab4dd323cf94c010321d8942fb2c1a01/addons/website_sale_digital/models/account_invoice.py#L14-L22



opw-3144600

closes odoo/odoo#112639

Signed-off-by: Morgane Demesmaeker <edm@odoo.com>