No behaviour changes, only migrate code and add
some docstring.

The 2 calendar notification routes were in auth
`none`. This causes no problem, but since they are
reimplementing the auth user mecanism, it is better
to set them directly in auth `user`. Moreover, they
are only called from webclient, when the user is
logged.

This commit also migrate them to new API (we forgot
them ....)

Closes #13054

* Remove dead code
* Clean variable assignation to a record set and use a dedicated variable to
  hold the result instead of setting a property on records

website_quote still calls sale_order_line.product_id_change() in old API,
giving arguments that are not supported anymore. It does not crash because
'reasons'. However the new onchange does not return values since v9. The
call is therefore not necessary anymore and the computation has been adapted
accordingly.

Write in models/account_invoice.py didn't handle multiple records.
Now fixed.