MDL-80160 admin: Fix site admins selector UI if the list is hardcoded

If the list of site admins was defined in the config.php, the UI to
select / modify admins never worked. But there was no indication of the
reason which was unexpected and confusing.

admin/roles/admins.php

@@ -38,6 +38,19 @@ if (!is_siteadmin()) {
 }
 
 $admisselector = new core_role_admins_existing_selector();
+
+if (array_key_exists('siteadmins', $CFG->config_php_settings)) {
+    echo $OUTPUT->header();
+    echo $OUTPUT->heading(get_string('manageadmins', 'core_role'), 3);
+    echo $OUTPUT->notification(get_string('siteadministratorsconfigphp', 'core_role'), \core\output\notification::NOTIFY_INFO);
+    echo $OUTPUT->box_start();
+    echo $OUTPUT->paragraph(get_string('existingadmins', 'core_role'));
+    $admisselector->display();
+    echo $OUTPUT->box_end();
+    echo $OUTPUT->footer();
+    die();
+}
+
 $potentialadmisselector = new core_role_admins_potential_selector();
 
 if ($addusersaction) {

lang/en/role.php

@@ -433,6 +433,7 @@ $string['showallroles'] = 'Show all roles';
 $string['showthisuserspermissions'] = 'Show this user\'s permissions';
 $string['site:accessallgroups'] = 'Access all groups';
 $string['siteadministrators'] = 'Site administrators';
+$string['siteadministratorsconfigphp'] = 'The list of site administrators can\'t be changed because it is defined in config.php.';
 $string['site:approvecourse'] = 'Approve course creation';
 $string['site:backup'] = 'Backup courses';
 $string['site:config'] = 'Change site configuration';