[QOLSVC-8437] add 'main' class to the appropriate tag to match latest…

… CKAN

- Commit ckan/ckan@f5e04e19806 changed the main tag from using 'role' to 'class',
we need to incorporate this so that styles are properly applied to its content

ckanext/data_qld/templates/page.html

@@ -16,7 +16,7 @@
 {% endblock %}
 
 {% block maintag %}
-    <div role="main" class={{ h.set_background_image_class() }}>
+    <div role="main" class="main {{ h.set_background_image_class() }}">
 {% endblock %}
 
 {% block flash %}
@@ -80,4 +80,4 @@ <h1>My page content</h1>
         </article>
         {% endblock %}
     </div>
-{% endblock %}
+{% endblock %}