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.

plugins/FBConnect/FBConnectPlugin.php

@@ -200,11 +200,13 @@ class FBConnectPlugin extends Plugin
              $action->menuItem(common_local_url('smssettings'),
                  _('Connect'), _('Connect to SMS, Twitter'), false, 'nav_connect');
             }
-            $action->menuItem(common_local_url('invite'),
-                _('Invite'),
-                sprintf(_('Invite friends and colleagues to join you on %s'),
-                common_config('site', 'name')),
-                false, 'nav_invitecontact');
+            if (common_config('invite', 'enabled')) {
+                $action->menuItem(common_local_url('invite'),
+                    _('Invite'),
+                    sprintf(_('Invite friends and colleagues to join you on %s'),
+                    common_config('site', 'name')),
+                    false, 'nav_invitecontact');
+            }
 
             // Need to override the Logout link to make it do FB stuff
             if ($flink && $fbuid > 0) {