Apply topic tag filters when viewing a single tag

Includes HTML updates to let user click into unfiltered view, when
viewing a single tag.

tildes/tildes/templates/home.jinja2

@@ -71,7 +71,7 @@
     <li>User settings</li>
 
     <ul class="nav">
-      {% if not (tag or unfiltered) %}
+      {% if not unfiltered %}
       <li><details>
         <summary>Filtered topic tags ({{ request.user.filtered_topic_tags|length }})</summary>
         <ul class="topic-tags">

tildes/tildes/templates/topic_listing.jinja2

@@ -112,6 +112,12 @@
     Showing only topics with the tag "{{ tag|replace('_', ' ') }}".
     <a href="{{ request.current_listing_normal_url() }}">Back to normal view</a>
   {% endif %}
+
+  {% if request.user.filtered_topic_tags and not unfiltered %}
+    <br>
+    Topic tag filters active (see sidebar).
+    <a href="{{ request.current_listing_normal_url({'tag': tag, 'unfiltered': 'true'}) }}">View unfiltered list</a>
+  {% endif %}
 {% elif unfiltered %}
   Showing unfiltered topic list.
   <a href="{{ request.current_listing_normal_url() }}">Back to normal view</a>

tildes/tildes/views/topic.py

@@ -213,11 +213,20 @@ def get_group_topics(  # noqa
     if after:
         query = query.after_id36(after)
 
-    # apply topic tag filters unless they're disabled or viewing a single tag
-    if request.user and request.user.filtered_topic_tags and not (tag or unfiltered):
+    # apply topic tag filters unless they're disabled
+    if request.user and request.user.filtered_topic_tags and not unfiltered:
+        filtered_topic_tags = request.user.filtered_topic_tags
+
+        # if viewing single tag, don't filter that tag and its ancestors
+        # for example, if viewing "ask.survey", don't filter "ask.survey" or "ask"
+        if tag:
+            filtered_topic_tags = [
+                ft for ft in filtered_topic_tags if not tag.descendant_of(ft)
+            ]
+
         query = query.filter(
             ~Topic.tags.descendant_of(  # type: ignore
-                any_(cast(request.user.filtered_topic_tags, TagList))
+                any_(cast(filtered_topic_tags, TagList))
             )
         )