[FIX] website: fix header vertical template with off canvas menu

Steps to reproduce the bug:

- Go to the website edit mode.
- Choose the 'vertical' template header.
- Select 'off-canvas' in the 'mobile menu' option of the navbar.
- Bug: On mobile view, the menu links are not clickable.

This is caused by the addition of the "order-first" class to the navbar
by commit [1], which causes the menu to be placed behind the backdrop in
mobile view when off-canvas is activated. This only happens with
Firefox, and there is likely a difference in how Chrome and Firefox
handle the "order" property based on the positioning of elements.

[1]: https://github.com/odoo/odoo/commit/2a000e33c5a44ddf0a777b43d8266cc413d8e4e2

opw-3009202

closes odoo/odoo#117683

X-original-commit: 7baa2d3e4949cf3c6e1b1130f03cc2b051dbff0f
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/static/src/scss/website.scss

@@ -1068,6 +1068,13 @@ header {
                     // menu when the off-canvas menu is open.
                     display: none;
                 }
+                #top_menu {
+                    // This is useful because otherwise, the 'order-first' class
+                    // of the 'template_header_vertical' template puts the menu
+                    // behind the "backdrop" and it's no longer possible to
+                    // click on the menu links (only on Firefox).
+                    order: unset !important;
+                }
             }
         }
         &.collapsing #top_menu {