1. Tracker running in `Public` or `Whitelisted` mode. 
   Downloaded torrents should never include any tracker key.
3. Tracker running in `Private` or `PrivateListed` mode.
   2.1 Guest user. 
       Downloaded torrents should never include any tracker key.
   2.2 Authenticated user
       2.2.1 Index can get the tracker user's key (from the Index database or generate new ones).
             - We should include the user's tracker key.
             - We should change the download torrent button label from `download torrent` to `download private torrent` (this is an Idex GUI issue).
       2.2.2 The user's tracker key is expired and the tracker API is unavailable (can't generate a new one).
             Return a `503 Service Unavailable` response with the text `Can't generate user's tracker key`.