Add back bootstrap_carousel

These were semi-removed in favor of starting over from scratch and refactoring how they were used.

To use:

    $slides = [];
    $slides[] = [
      'image' => ['#theme' => 'image', '#uri' => 'http://lorempixel.com/1000/400/?1'],
      'title' => t('Title Slide 1'),
      'description' => t('Description Slide 1'),
    ];
    $slides[] = [
      'image' => ['#theme' => 'image', '#uri' => 'http://lorempixel.com/1000/400/?2'],
      'title' => t('Title Slide 2'),
      'description' => t('Description Slide 2'),
    ];
    $slides[] = [
      'image' => ['#theme' => 'image', '#uri' => 'http://lorempixel.com/1000/400/?3'],
      'title' => t('Title Slide 3'),
      'description' => t('Description Slide 3'),
    ];
    $build['carousel'] = [
      '#theme' => 'bootstrap_carousel',
      '#attributes' => [],
      '#controls' => TRUE,
      '#id' => NULL,
      '#indicators' => TRUE,
      '#interval' => 5000,
      '#pause' => 'hover',
      '#slides' => $slides,
      '#start_index' => 0,
      '#wrap' => TRUE,
    ];