[FIX] website: show mega menu in + (extra) menu

The mega-menu inside "+" menu (that contains items that do not fit on
one line) could not be used at all.

This is because we only managed normal dropdown-menu (ul) and not
mega menu (div.o_mega_menu).

But once the menu is shown, there is an issue because the two main
widgets are column on "lg" breakpoint, and inside the menu there is no
width but we might still be on lg breakpoint.

So this commit also try to reset col values (max-width and flex) as is
the case in mobile view (because "col-lg" are not applied).

opw-2313019
closes #56367

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>