Topic: add property to decide if user is "source"

This isn't meaningful yet, but we'll attach more logic shortly.

tildes/tildes/models/topic/topic.py

@@ -448,3 +448,8 @@ class Topic(DatabaseModel):
                 return f"<cite>@{authors[0]}:</cite><blockquote>{tweet}</blockquote>"
 
         return None
+
+    @property
+    def is_user_treated_as_source(self) -> bool:
+        """Return whether the user that posted the topic is its "source"."""
+        return self.is_text_type

tildes/tildes/templates/macros/topics.jinja2

@@ -79,9 +79,9 @@
         </a>
       </div>
 
-      {% if topic.is_text_type %}
+      {% if topic.is_user_treated_as_source %}
         <div aria-label="Posted by">{{ username_linked(topic.user.username) }}</div>
-      {% elif topic.is_link_type %}
+      {% else %}
         <div aria-label="Link domain">{{ topic.link_domain }}</div>
       {% endif %}