title: Create amp-form inside amp-lightbox
body: <p>I am trying to submit a form inside lightbox modal. But popup showing empty content. my code is as follows:</p>
  
  <pre><code>&lt;amp-lightbox id="success-lightbox" layout="nodisplay"&gt;
      &lt;amp-list src="http://localhost:4000" width="auto" height="200" layout="fixed-height" items="." single-item&gt;
          &lt;template type="amp-mustache"&gt;
              &lt;div class="lightbox-modal"&gt;
                  &lt;form action="/" method="get" target="_top"&gt;
                      {{#items}}
                      &lt;input type="checkbox" value={{id}} /&gt; {{/items}}
                      &lt;input type="submit" value="submit" /&gt;
                  &lt;/form&gt;
              &lt;/div&gt;
          &lt;/template&gt;
      &lt;/amp-list&gt;
  &lt;/amp-lightbox&gt;
  &lt;input class="" type="submit" value="show more" on="tap:success-lightbox" /&gt;
  </code></pre>
  
  <p>I included all dependencies required <code>amp-form</code>, <code>amp-lightbox</code>, <code>amp-list</code>, <code>amp-mustache</code>.
  If I <strong>remove</strong> the form tag, checkboxes are showing fine.</p>
  
  <p>The working code:</p>
  
  <pre><code>&lt;amp-lightbox id="success-lightbox" layout="nodisplay"&gt;
      &lt;amp-list src="http://localhost:4000" width="auto" height="200" layout="fixed-height" items="." single-item&gt;
          &lt;template type="amp-mustache"&gt;
              &lt;div class="lightbox-modal"&gt;
                  {{#items}}
                  &lt;input type="checkbox" value={{id}} /&gt; {{/items}}
                  &lt;input type="submit" value="submit" /&gt;
              &lt;/div&gt;
          &lt;/template&gt;
      &lt;/amp-list&gt;
  &lt;/amp-lightbox&gt;
  &lt;input class="" type="submit" value="show more" on="tap:success-lightbox" /&gt;
  </code></pre>
  
  <p>I went through docs. but didn't find anything. is it not possible to nest the form inside amp-lightbox ?? if not possible, is there any workaround to submit data from lightbox ??</p>