[FIX] portal: <body/> should still be the scrolling element

Commit https://github.com/odoo/odoo/commit/a7a89f31741c3df1d15f1fb8d0433b2761a7e1d0
was a bit too optimistic. Too many features still depend on the scroll
event listened on the <body/> element.

addons/portal/static/src/scss/portal.scss

@@ -17,8 +17,7 @@ $-navbar-height: $nav-link-height !default;
     display: flex;
     flex-flow: column nowrap;
     width: 100%;
-    height: 100%;
-    overflow: auto;
+    min-height: 100%;
 
     > * {
         flex: 0 0 auto;