Set sale/purchase uom when activating a product for sale/purchase
When marking and existing product as salable, thee sale uom is empty and the user is forced to manually enter one. On the other hand, if you set the default_uom of a salable product, this uom is used as sale uom.
For consistency I think that the default_uom should be set as sale_uom when marking the salable field.
The same applies for purchase.