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.
Martin Trigaux
3695886531