forum: properly redirect topic after changing forum

The URL changes so using the referrer is not much of an option.

app/routes/posts/edit.py

@@ -79,7 +79,11 @@ def edit_post(postid):
         for a, file in attachments:
             a.set_file(file)
 
-        return redirect(referrer)
+        # Determine topic URL now, in case forum was changed
+        if isinstance(p, Topic):
+            return redirect(url_for('forum_topic', f=p.forum, page=(p,1)))
+        else:
+            return redirect(referrer)
 
     # Non-submitted form
     if isinstance(p, Comment):