message: Add optional className to download button.
diff: @@ -38,10 +38,10 @@ export class DownloadButton extends SafetyFirst<DownloadButtonProps, DownloadBut
  }
  render () {
  - const { filename } = this.props;
  + const { className, filename } = this.props;
  const { error, inFlight } = this.state;
  // The position styling and always-hidden filename are so the button doesn't resize when its content changes.
  - return <div>
  + return <div className={className}>
  <button className="btn btn-primary" style={buttonStyle} disabled={inFlight} type="button" onClick={() => this.download()}>
  <i className="fa fa-fw fa-download" />&nbsp;Download
  <span style={{position: 'relative'}}>
  @@ -59,6 +59,7 @@ export class DownloadButton extends SafetyFirst<DownloadButtonProps, DownloadBut
  export type DownloadButtonProps = {
  url: string,
  filename?: string,
  + className?: string,
  };
  export type DownloadButtonState = {
  