menu: always show logout entry

"index.php*" match in dashboard privileges overlays a bit
with "index.php?logout" so that we'll have to hardcode an
exception for now.  Other approaches to be taken here mid-
term are 'visibility="always"' in Menu.xml or restructure
the logout link under /ui/logout or /logout.php.

PR: https://forum.opnsense.org/index.php?topic=3431.0

src/opnsense/mvc/app/models/OPNsense/Core/ACL.php

@@ -225,6 +225,11 @@ class ACL
      */
     public function isPageAccessible($username, $url)
     {
+        if ($url == '/index.php?logout') {
+            // always allow logout, could use better structuring...
+            return true;
+        }
+
         if (array_key_exists($username, $this->legacyUsers)) {
             // search user privs
             foreach ($this->legacyUsers[$username]["priv"] as $privset) {