MDL-72119 admin: Move Enable messaging to Advanced features

The "Enable messaging" setting has been moved from "Messaging
settings" page to "Advanced features".
Besides, the "Messaging settings" page won't be displayed if
$CFG->messaging is disabled.

admin/settings/messaging.php

@@ -24,11 +24,12 @@
 defined('MOODLE_INTERNAL') || die();
 
 if ($hassiteconfig) {
-    $temp = new admin_settingpage('messages', new lang_string('messagingssettings', 'admin'));
-    $temp->add(new admin_setting_configcheckbox('messaging',
-        new lang_string('messaging', 'admin'),
-        new lang_string('configmessaging', 'admin'),
-        1));
+    $temp = new admin_settingpage('messages',
+        new lang_string('messagingssettings', 'admin'),
+        'moodle/site:config',
+        !$CFG->messaging
+    );
+
     $temp->add(new admin_setting_configcheckbox('messagingallusers',
             new lang_string('messagingallusers', 'admin'),
             new lang_string('configmessagingallusers', 'admin'),

admin/settings/subsystems.php

@@ -58,6 +58,12 @@ if ($hassiteconfig) { // speedup for non-admins, add all caps used on this page
         1)
     );
 
+    $optionalsubsystems->add(new admin_setting_configcheckbox('messaging',
+        new lang_string('messaging', 'admin'),
+        new lang_string('configmessaging', 'admin'),
+        1)
+    );
+
     $fullunicodesupport = true;
     if ($DB->get_dbfamily() == 'mysql') {
         $collation = $DB->get_dbcollation();