[FIX] portal, website: make affix menu appear from *under* the odoo topbar

addons/portal/static/src/less/website.ui.less

@@ -11,7 +11,7 @@ body.o_connected_user {
 #oe_main_menu_navbar {
     .o-position-absolute(0, 0, auto, 0);
     position: fixed;
-    z-index: 1001;
+    z-index: @zindex-modal - 10;
 
     font-family: @o-we-font-family;
     font-size: @odoo-font-size-base;

addons/website/static/src/less/website.snippets.less

@@ -456,7 +456,7 @@ div.carousel {
 #wrapwrap .o_header_affix {
     .o-position-absolute(0, 0, auto, 0);
     position: fixed;
-    z-index: @zindex-modal - 1;
+    z-index: @zindex-modal - 20;
     background: @navbar-default-bg;
     border-bottom: @navbar-default-border;