[FIX] web: allow to open community website menu on iOS

Before this commit, the menu to go into the backend from the website was
not possible to open on iOS without opening the right menu first. This
was because position: fixed is not able to counter overflow: hidden in
iOS. This is the fix for stable versions, the proper solution will be
done in master.

Closes https://github.com/odoo/odoo/issues/29390
task-1923151

closes odoo/odoo#30507

addons/web/static/src/less/navbar.less

@@ -94,7 +94,6 @@
 
         position: relative;
         height: @odoo-navbar-height;
-        overflow: hidden;
 
         > ul {
             > li {
@@ -110,6 +109,7 @@
 
             &.o_menu_sections {
                 width: 100%;
+                display: none;
 
                 > li.open .dropdown-menu {
                     position: static;
@@ -158,6 +158,10 @@
         .o_main_navbar {
             height: 100%;
             overflow: auto;
+
+            .o_menu_sections {
+                display: block;
+            }
         }
     }
 }