[FIX] website_sale_digital: allow download without reconciliation

TLDR: after installation accounting app, invoices paid online get status "In Payment" until
reconciliation; this commit allows to download digital content without waiting
reconciliation which may take days or week

Steps:

* Install Accounting app
* Install website_sale_digital
* Create Digital product (Product Type: Service + use Digital Files smart button
to upload a file)
* Setup payment acquirer (e.g. Paypal)
* Buy digital product online and pay
* Open the Sale Order in Portal

Before:

* No "Downloads" button near to product name

After

* User can download files after purchasing

Notes

* In Odoo 13.0 it worked via "post_at" option, which is removed in Odoo 14. See
https://github.com/odoo/odoo/commit/caeb782841fc5a7ad71a196e2c9ee67644ef9074

:

  > - Remove the post_at bank reconciliation feature. The "paid" state will be set on the invoices only if reconciled with a journal entry involving the journal's liquidity account.
  >     With invoicing, the payment will do that so the "in_payment" state should never be shown up.
  >     With accounting, only the statement lines have the power to move an invoice to the "paid" state.

---

opw-2353064

closes odoo/odoo#59825

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>