forum: do not delete threads of promoted topics

Threads of promoted topics belong to the programs they were promoted
into, so they shouldn't be deleted.

app/models/topic.py

@@ -54,7 +54,8 @@ class Topic(Post):
 
     def delete(self):
         """Recursively delete topic and all associated contents."""
-        self.thread.delete()
+        if self.promotion is None:
+            self.thread.delete()
         db.session.delete(self)
 
     def __repr__(self):