Sometimes `Jobs::PushNotification` gets stuck, probably because of the
network call. This commit replaces `Excon` with `FinalDestination::HTTP`
which is safer.
This change ensures native push notifications respect the site setting for push_notification_time_window_mins. Previously only web push notifications would account for the delay, now we can bring more consistency between Discourse in browser vs Hub, by applying the same delay strategy to both forms of push notifications.