[FIX] account: avoid losing product on invoice

Add restriction on product_id field to prevent the suppression of the product
if already present in an invoice.
This is to avoid the suppression of a used product variant when modifying
the list of attributes.#
Due to the constrain, the variant will be disabled instead of deleted.
Fixes #4129

Add warning message on the product form to warn users about the potential impact
of modifying variants.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32