Cleanup current_version and latest_version

This updates the logic so that only apps with a status in
`amo.WEBAPPS_APPROVED_STATUSES` have a `current_version` since a
`current_version` signifies the publicly available version of an app.
Yeah, I shouldn't be so lazy. It doesn't matter as long as something is passed in and it doesn't try to use `current_version` to get the file_obj.