static setSlideNumber(number) {
    if (Hash.getSlideNumber() !== (number - 1)) {
      history.pushState({
        slideI: number - 1
      }, `Slide ${number}`, `${HASH}=${number}`);
    }
  }
It will update the hash (if it's different) so it reflects the slide
number being visible.
@param {number} number The number of the slide we're transitioning to.