Use serializers for project / group boards
Currently we manually serialize boards in a few places using object.as_json
(i.e. https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/controllers/ee/boards/boards_controller.rb#L77-84 and a few places in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2912). Ultimately we should also be using serializers in order to accomplish this.