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.
Confusing method names are confusing... hopefully what's "visible", "public" and "published" will be made clearer once your hidden apps change land.