Add custom event if download was successful

[erikos]

To further follow the funnel if and when we are loosing customers along the way we would like to know when the download of the content pack was successful. An event should be triggered when the user sees the success screen.

Not sure another extra event should be triggered when the user lands on the Discovery page for the first time or if this is good enough. The goal here is to find out how many of the users have been successfully downloading a content pack.