rename ads -> offers, add new listing (syd)

src/index.njk

@@ -21,10 +21,11 @@ title: FediLearns Classifieds
         <h2>{{ tag }}</h2>
         <p>{{ tag | getTagline }}</p>
       </div>
-      {% if collections[tag].length == 0 %}
+      {% set listings = collections[tag] | filterByTag("offers") %}
+      {% if listings | length == 0 %}
       <p>No listings.</p>
       {% endif %}
-      {% for listing in collections[tag] | sortAlphabeticallyByTitle %}
+      {% for listing in listings | sortAlphabeticallyByTitle %}
         {% include "listing.njk" %}
       {% endfor %}
     </div>