For code review - don't merge

You probably don't want a "find" here. find will raise an error if it fails to find. You could probably just do
```
def current_user
  User.find_by_id(session[:user_id])
end
```

That way if the session user id is nil it just returns nil. If it's set but doesn't map to a real user you get nil. You shouldn't ever get an error.