[#730] Only show 'Popular' sort option if tracking enabled

Sorting datasets by popularity only works if page view tracking is
enabled, so don't show the 'Popular' option in the 'Order by:' dropdown
if tracking isn't enabled.

ckan/templates/snippets/sort_by.html

@@ -15,7 +15,9 @@
     <option value="title_string asc"{% if sort=='title_string asc' %} selected="selected"{% endif %}>{{ _('Name Ascending') }}</option>
     <option value="title_string desc"{% if sort=='title_string desc' %} selected="selected"{% endif %}>{{ _('Name Descending') }}</option>
     <option value="metadata_modified desc"{% if sort=='metadata_modified desc' %} selected="selected"{% endif %}>{{ _('Last Modified') }}</option>
-    <option value="views_recent desc"{% if sort=='views_recent desc' %} selected="selected"{% endif %}>{{ _('Popular') }}</option>
+    {% if g.tracking_enabled %}
+      <option value="views_recent desc"{% if sort=='views_recent desc' %} selected="selected"{% endif %}>{{ _('Popular') }}</option>
+    {% endif %}
   </select>
   <button class="btn js-hide" type="submit">{{ _('Go') }}</button>
 </span>