Release 6.20.0: Configuration value telling the UI to show OLOX2 downloads

[changerequest]

2 values are added to the preferences tree:

/modules/olox20/active ? Tells the UI if the user is allowed to use the OXtender for Microsoft Outlook 2.

/modules/olox20/module ? Is set to false to prevent the UI from trying to load a plugin.

See http://oxpedia.org/wiki/index.php?title=HTTP_API#Module_.22config.22