Move the downloads page in the top level menu

uitwww/navbar.json

 [
-    {
-        "name": "Home page",
-        "endpoint": "pages.index",
-        "subs": [
     {
         "name": "Download",
-                "endpoint": "pages.download_index"
-            }
-        ]
+        "endpoint": "pages.download_index",
+        "subs": []
     },
     {
         "name": "Scopri Ubuntu",