Add subgroups list in group sidebar (fixes #567)

tildes/tildes/templates/topic_listing.jinja2

@@ -224,6 +224,17 @@
     {{ group.sidebar_rendered_html|safe }}
   {% endif %}
 
+  {% if subgroups %}
+    <ul class="nav">
+      <li>Subgroups</li>
+      <ul class="nav">
+      {% for subgroup in subgroups %}
+        <li class="nav-item">{{ group_linked(subgroup.path) }}</li>
+      {% endfor %}
+      </ul>
+    </ul>
+  {% endif %}
+
   {% if wiki_pages or wiki_has_index %}
     <ul class="nav">
       <li>Group wiki pages</li>

tildes/tildes/views/topic.py

@@ -157,10 +157,17 @@ def get_group_topics(
             groups = [
                 group for group in request.query(Group).all() if group.path != "test"
             ]
+        subgroups = None
     else:
         # otherwise, just topics from the single group that we're looking at
         groups = [request.context]
 
+        subgroups = [
+            group
+            for group in request.query(Group).all()
+            if group.is_subgroup_of(request.context)
+        ]
+
     default_settings = _get_default_settings(request, order)
 
     if order is missing:
@@ -245,6 +252,7 @@ def get_group_topics(
         "unfiltered": unfiltered,
         "wiki_pages": wiki_pages,
         "wiki_has_index": wiki_has_index,
+        "subgroups": subgroups,
     }