navbar: Add login and signup buttons for spectators.

Remove login buttons from right sidebar and add them to navbar
so that the user instantly notices them.

static/js/tippyjs.js

@@ -226,6 +226,7 @@ export function initialize() {
             "#user_filter_icon",
             "#scroll-to-bottom-button-clickable-area",
             ".code_external_link",
+            ".spectator_narrow_login_button",
         ],
         appendTo: () => document.body,
     });