templates: add an id attribute to cells that have a slug (#20986)

combo/public/templates/combo/placeholder.html

@@ -2,6 +2,7 @@
 {% if render %}
 {% for cell in cells %}
 <div class="cell {{ cell.css_class_names }} {% if cell.slug %}{{cell.slug}}{% endif %} {% if cell|shown_because_admin:request %}shown-because-admin{% endif %}"
+     {% if cell.slug %}id="{{ cell.slug }}"{% endif %}
      data-ajax-cell-url="{{ site_base }}{% url 'combo-public-ajax-page-cell' page_pk=cell.page.id cell_reference=cell.get_reference %}"
      data-ajax-cell-loading-message="{% trans "Loading..." %}"
      {% if cell.ajax_refresh %}