title: Add custom class from link to drupal-modal drupal 8 with bootstrap theme
body: <p>In Drupal 8, with the bootstrap theme when you create a link with <code>class</code> and <code>data-dialog-type</code> attributes like the bellow code: </p>
  
  <pre><code>&lt;a class="use-ajax" data-dialog-type="modal"
    href="http://drupal.page/front"&gt;text
  &lt;/a&gt;
  </code></pre>
  
  <p>You will open content of the page in <code>#drupal-modal</code> element that has these html wrappers:</p>
  
  <pre><code>&lt;div id="drupal-modal" class="modal fade in" tabindex="-1" role="dialog" style="display: block;"&gt;
      &lt;div class="modal-dialog" role="document"&gt;
           &lt;div class="modal-content"&gt;
  </code></pre>
  
  <p>This structure is generated in: <code>\themes\bootstrap\js\modal.js</code> how we can see on the <a href="https://gist.github.com/Yog/a9b899ca0b31cc39f5929a75d5317dd1" rel="nofollow noreferrer">link</a>.</p>
  
  <p>How do I modify it so that I can pass a class name to the <code>#drupal-modal</code> element from the link <code>a.use-ajax</code>? The class name text could be value of an attribute of the link.  </p>
  
  <p>Specifically I'd like to add <code>modal-lg</code> or <code>modal-sm</code> classes or some custom ones. </p>