[FIX] web_kanban: parent can be undefined in some cases

opw-611106, when clicking quickly Sales > Opportunities than Sales > Leads.
[FIX] purchase: now allow 'can be purchased' to be checked even if not variant

Before, the field purchase_ok was set as readonly on the product form if the product wasn't a variant, to avoid users to set all the products variants associated to the product template to be set as purchase able in one action.

We now consider that this can be done by the end user