Prevent adding a TopicLog entry if tags unchanged

Previously, an entry would be added to the topic log even if someone
didn't actually change the topic's tags at all. This commit fixes that,
as well as moving the the template fragment that renders the list into a
separate file in includes/ that can be used both for the initial render
as well as by intercooler when it updates that section of the page.

tildes/tildes/templates/includes/topic_tags.jinja2

@@ -0,0 +1,9 @@
+<ul class="topic-tags">
+  {% for tag in topic.tags %}
+    <li class="label label-topic-tag">
+      <a href="/~{{ topic.group.path }}?tag={{ tag.replace(' ', '_') }}">{{ tag }}</a>
+    </li>
+  {% else %}
+    <li class="label label-topic-tag">No tags</li>
+  {% endfor %}
+</ul>

tildes/tildes/templates/intercooler/topic_tags.jinja2

@@ -1,5 +1 @@
-<ul class="topic-tags">
-  {% for tag in topic.tags %}
-    <li class="label label-topic-tag">{{ tag }}</li>
-  {% endfor %}
-</ul>
+{% include 'includes/topic_tags.jinja2' %}

tildes/tildes/templates/topic.jinja2

@@ -204,15 +204,7 @@
 <dl>
   <dt>Tags</dt>
   <dd>
-    <ul class="topic-tags">
-      {% for tag in topic.tags %}
-        <li class="label label-topic-tag">
-          <a href="/~{{ topic.group.path }}?tag={{ tag.replace(' ', '_') }}">{{ tag }}</a>
-        </li>
-      {% else %}
-        <li class="label label-topic-tag">No tags</li>
-      {% endfor %}
-    </ul>
+    {% include 'includes/topic_tags.jinja2' %}
   </dd>
 
   <dt>Comments</dt>

tildes/tildes/views/api/web/topic.py

@@ -14,7 +14,7 @@ from tildes.models.log import LogTopic
 from tildes.models.topic import Topic, TopicVote
 from tildes.schemas.group import GroupSchema
 from tildes.schemas.topic import TopicSchema
-from tildes.views import IC_EMPTY, IC_NOOP
+from tildes.views import IC_NOOP
 from tildes.views.decorators import ic_view_config
 
 
@@ -160,28 +160,23 @@ def tag_topic(request: Request, tags: str) -> dict:
     """Apply tags to a topic with Intercooler."""
     topic = request.context
 
-    if not tags:
-        request.db_session.add(
-            LogTopic(
-                LogEventType.TOPIC_TAG,
-                request,
-                topic,
-                info={'old': topic.tags, 'new': []},
-            ),
-        )
-        topic.tags = []
-        return IC_EMPTY
-
-    # split the tag string on commas
-    split_tags = tags.split(',')
+    if tags:
+        # split the tag string on commas
+        new_tags = tags.split(',')
+    else:
+        new_tags = []
 
     old_tags = topic.tags
 
     try:
-        topic.tags = split_tags
+        topic.tags = new_tags
     except ValidationError:
         raise ValidationError({'tags': ['Invalid tags']})
 
+    # if tags weren't changed, don't add a log entry or update page
+    if set(topic.tags) == set(old_tags):
+        return IC_NOOP
+
     request.db_session.add(
         LogTopic(
             LogEventType.TOPIC_TAG,