[#1996] Fix featured group link.

URL was previously set as a jinja template variable, but was being
called inside a child block of the template, so the var was out of
scope and coming up empty. See:
http://jinja.pocoo.org/docs/dev/templates/#block-nesting-and-scope

ckan/templates/snippets/group_item.html

@@ -2,17 +2,16 @@
   <section class="group-list module module-narrow module-shallow">
     {% block group_item_header %}
       <header class="module-heading">
-        {% set url=h.url_for(controller='group', action='read', id=group.name) %}
         {% set truncate=truncate or 0 %}
         {% set truncate_title = truncate_title or 0 %}
         {% set title = group.title or group.name %}
         {% block group_item_header_image %}
-          <a class="module-image" href="{{ url }}">
+          <a class="module-image" href="{{ h.url_for(controller='group', action='read', id=group.name) }}">
             <img src="{{ group.image_display_url or h.url_for_static('/base/images/placeholder-group.png') }}" alt="{{ group.name }}" />
           </a>
         {% endblock %}
         {% block group_item_header_title %}
-          <h3 class="media-heading"><a href="{{ url }}">{{ group.title or group.name }}</a></h3>
+          <h3 class="media-heading"><a href="{{ h.url_for(controller='group', action='read', id=group.name) }}">{{ group.title or group.name }}</a></h3>
         {% endblock %}
         {% block group_item_header_description %}
           {% if group.description %}