[FIX] web: handle auto margins for auto more menus
[FIX] web: handle auto margins for auto more menus

The auto more menu feature is used to enforce that all items of a menu
stay on one line by adding the ones that would overflow in a dropdown
menu. The feature first need to compute the maximum width the menu can
have. This computation was not correct as it was not considering auto
margins.

This is particularly important with BS4 where all standard menus have
auto margins.

closes odoo/odoo#28683
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo