Adds $config['invite']['enabled'] to enable/disable invites.

The invite function may not applicable for private and/or closed installs. This adds a configuration option to enable/disable invites (defaulting to enabled), hides the "Invite" nav item when necessary, and adds a check to actions/invite.php.

Note that I haven't tried the Facebook application so I didn't add any checks to actions/facebookinvite.php.

lib/common.php

@@ -130,6 +130,8 @@ $config =
               'host' => null, # only set if != server
               'debug' => false, # print extra debug info
               'public' => array()), # JIDs of users who want to receive the public stream
+        'invite' =>
+        array('enabled' => true),
         'sphinx' =>
         array('enabled' => false,
               'server' => 'localhost',