Don't check moderated_by field for rollback

The moderated_by field is pretty strange (see 2298), let's not use it
to check if we can rollback to a version
931: added teams permission checks to the rollback button

We only allow users to rollback if they can add a new version to the
language.
931: fix rollback display logic for the diffing page

Now it matches the logic for when we show the rollback button on the
page for the subtitle version.