Don't allow un-publishing a post

It was previously possible to take a previously public post and move it back to a draft, which shouldn't be possible - once a post is live, that should be it.

This requires we add a bit of logic when performing updates to make sure that if the post-status was previously published, it can only ever be set to published.

If I want to do this, it should be done via a delete operation.

Because we're now interacting with the mf2 object, we need to amend updateIteratesThroughActionsAndReturnsFinalValue to pass in a real object.