Proper fix for duplicate 'sidebar' div ID

The initial attempt at fixing this issue [1] introduced a regression in
the persistence of the sidebar's collapse state.

Using a different ID for the toggle button or the sidebar div caused
other issues (e.g. malfunctioning hambuger menu on narrow screens, or
failure to save collapse state).

The root cause was a bug in the javascript code saving the sidebar's
state, which was referencing the toggle button's id instead of the
sidebar div's.

We now have ID 'sidebar' for the actual sidebar, and 'sidebar-btn' for
the toggle button, and the javascript has been modified accordingly.

Fixes #24976

[1]: commit b7b9145

core/layout_api.php

@@ -946,7 +946,7 @@ function layout_sidebar_end() {
 
 	$t_collapse_block = is_collapsed( 'sidebar' );
 
-	echo '<div id="sidebar" class="sidebar-toggle sidebar-collapse">';
+	echo '<div id="sidebar-btn" class="sidebar-toggle sidebar-collapse">';
 	if( layout_is_rtl() ) {
 		$t_block_icon = $t_collapse_block ? 'fa-angle-double-left' : 'fa-angle-double-right';
 		echo '<i data-icon2="ace-icon fa fa-angle-double-left" data-icon1="ace-icon fa fa-angle-double-right"

js/common.js

@@ -80,8 +80,8 @@ $(document).ready( function() {
         SetCookie( "collapse_settings", t_cookie );
     });
 
-    $('#sidebar.sidebar-toggle').on('click', function (event) {
-        var t_id = $(this).attr('id');
+    $('#sidebar-btn.sidebar-toggle').on('click', function (event) {
+        var t_id = $(this).closest('.sidebar').attr('id');
         var t_cookie = GetCookie("collapse_settings");
         if (1 == g_collapse_clear) {
             t_cookie = "";