make 'admin' a safe user name

'admin' is a pretty common username that people try when installing;
it was blacklisted because all of our admin panels were at /admin/*,
which would conflict with the admin user's namespace.

Changed the location of all admin panels to /panel/*, blacklisted the
nickname 'panel', and allowed 'admin'. Tested with a fresh install;
seems to work great.

plugins/Facebook/FacebookPlugin.php

@@ -105,7 +105,7 @@ class FacebookPlugin extends Plugin
      */
     function onStartInitializeRouter($m)
     {
-        $m->connect('admin/facebook', array('action' => 'facebookadminpanel'));
+        $m->connect('panel/facebook', array('action' => 'facebookadminpanel'));
 
         if (self::hasKeys()) {
             // Facebook App stuff