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/BitlyUrl/BitlyUrlPlugin.php

@@ -184,7 +184,7 @@ class BitlyUrlPlugin extends UrlShortenerPlugin
      */
     function onRouterInitialized($m)
     {
-        $m->connect('admin/bitly',
+        $m->connect('panel/bitly',
                     array('action' => 'bitlyadminpanel'));
         return true;
     }