[10.0][MIG] website_sale_unsaleable_options

[reinka]

This PR covers the migration to v10 and actually a [FIX] which is explained in my docstring of the controller:

[...]
optional_product_ids is saved on product.template.
However, the AJAX request sends the product_id saved
on product.product! Since these 2 IDs can vary, we have
to consider such cases here.
[...]

In my case when I choose Apple Wirless Keyboard as an optional product for the iMac, there was unexpected behaviour: The product_id sent via AJAX request is based on the product.product ID, which was 19, while the corresponding product.template ID was 16 (if I recall correctly). So this lead to the effect, that choosing the keyboard in the frontend didn't lead to it being recognised as an optional product. I fixed this by "converting" the product.product ID to its corresponding product.template ID.
I didn't check if this bug is also existent in earlier Odoo versions.

@pedrobaeza Somehow this PR also contains my commits from earlier today. I proceeded accordingly to your comment on my website_sale_order_company PR. Not sure if I did something wrong or this behaviour is normal.

[github-actions]

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