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.
You're also checking for `disabled_by_user` in the code below, making it exactly the same as `is_public` - which part is right ? the docs or the code ? :)