I would prefer to have `S3FrontApi` return `Future`s

For the record, we will have to call `Authenticated.async` anyway. I am going to move `Authenticated.apply` back to `AuthAction`. I think there is more work to be done around this since the new play loves `Future`, and the other endpoints aren't calling `async` but are calling `apply`, but I wouldn't want to see this pull request get any bigger!