[FIX] website_sale: accessory_product_ids are product.product, not product.template

This many2many field should actually have product.template as relation
But, in a stable release, we cannot alter the database
Therefore, when displaying accessory products in the card, it display the product_tmpl_id of the product.product, not itself
This relation should be changed in trunk
+ Actually displaying this field in the backend
[FIX] website_sale: backport commit 3cc4785ba6d27475323a606fef9466286024ad0e of master

This fix aimed saas-5, not master.
original Commit message:
display accessory field in form view and display the good accessory product inside ecommerce
This is related to previous rev 86055fb8cd5f86d9b4416d9d9a1a736e1c06aba9