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.
I actually removed this method locally. It's the same as `is_public`. But I'll add it back when I add hidden apps where there will be a difference between public and published. Public being accessible by anonymous users. Published meaning published to search and listing pages. (or other names if that's too confusing)