Messagebox PR followup:

 * Renamed /api/content/:id/commments to /api/content/:id/messages
 * Renamed /api/discussions/:id to /api/discussion/:id
 * Default visibility is retrieved from the config
   when the visibility has been omitted in createDiscussion
 * Cleaned up oae-discussion/activity.js file
 * Not retrieving parent message for activities when
   it has been deleted