MDL-74026 admin: consistently show search element on admin pages.

lib/adminlib.php

@@ -8804,10 +8804,11 @@ function admin_externalpage_setup($section, $extrabutton = '', array $extraurlpa
     $adminroot = admin_get_root(false, false); // settings not required for external pages
     $extpage = $adminroot->locate($section, true);
 
+    $hassiteconfig = has_capability('moodle/site:config', context_system::instance());
     if (empty($extpage) or !($extpage instanceof admin_externalpage)) {
         // The requested section isn't in the admin tree
         // It could be because the user has inadequate capapbilities or because the section doesn't exist
-        if (!has_capability('moodle/site:config', context_system::instance())) {
+        if (!$hassiteconfig) {
             // The requested section could depend on a different capability
             // but most likely the user has inadequate capabilities
             print_error('accessdenied', 'admin');
@@ -8878,6 +8879,13 @@ function admin_externalpage_setup($section, $extrabutton = '', array $extraurlpa
     $PAGE->set_title("$SITE->shortname: " . implode(": ", $visiblepathtosection));
     $PAGE->set_heading($SITE->fullname);
 
+    if ($hassiteconfig) {
+        $PAGE->add_header_action($OUTPUT->render_from_template('core_admin/header_search_input', [
+            'action' => new moodle_url('/admin/search.php'),
+            'query' => $PAGE->url->get_param('query'),
+        ]));
+    }
+
     // prevent caching in nav block
     $PAGE->navigation->clear_cache();
 }