Generate burn after reading link for pads

www/common/outer/invitation.js

@@ -17,6 +17,7 @@ var factory = function (Util, Cred, Nacl) {
         };
     };
 
+    // XXX move this function?
     Invite.generateSignPair = function () {
         var ed = Nacl.sign.keyPair();
         return {