Chat Federation: using S2S if available.

* if both local and remote instance have external XMPP connections enabled, the user joins the remote room with his local account
* some code refactoring (builtin.ts)

Note: documentation and settings descriptions are to do.

Related to #112

server/lib/federation/sanitize.ts

@@ -36,6 +36,11 @@ function sanitizePeertubeLiveChatInfos (chatInfos: any): LiveChatJSONLDAttribute
         url: link.url
       })
     }
+    if (link.type === 'xmpp-s2s') {
+      r.links.push({
+        type: link.type
+      })
+    }
   }
   return r
 }