Handle unpublishing featured actions better

[esoterik]

Right now, if you unpublish an action page that is featured, this callback runs:

def no_drafts_on_homepage                                                                                                           
  FeaturedActionPage.where(action_page_id: id).destroy_all unless published?   
end                                                                            

This means that fewer than 4 actions end up being featured, until someone edits the featured actions again. I'm not sure what the best way to fix this is, but it's not intuitive right now.