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.
reviewer tools are all about the latest version. And in this case current == latest. To me it's clearer to always use latest in mkt/reviewers/.