function renderTrack (common, key) {
  // Audio metadata: track-number
  if (common[key] && common[key].no) {
    let str = `${common[key].no}`
    if (common[key].of) {
      str += ` of ${common[key].of}`
    }
    const style = { textTransform: 'capitalize' }
    return (
      <div className={`audio-${key}`}>
        <label style={style}>{key}</label> {str}
      </div>
    )
  }
}
Render track or disk number string
@param common metadata.common part
@param key should be either 'track' or 'disk'
@return track or disk number metadata as JSX block