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.
The file update triggers signals which bump the counts of index_webapps. This one was actually strange b/c the reset_mock didn't work and it said it was called twice. I put a `pdb` there and it was only called once. Adding the `_signal=False` fixed it and also avoids some calls we don't care about.