title: Exclude the_post_thumbnail from gallery shortcode
body: <p>I am using this code to have a simple gallery on the page:</p>
  
  <pre><code>&lt;?php echo do_shortcode('[gallery itemtag="ul" icontag="li" size="full" columns="0" link="file" ]'); ?&gt;
  </code></pre>
  
  <p>The problem now is that the end-user has to upload an image via the Media page before selecting this image as featured image. </p>
  
  <p>I know this could be solved by adding the featured image's ID to the shortcode's exclude list, but how to get this ID automatically?</p>