Properly sort by newest reply

The previous algorithm would fail with this order of comments:
- Comment 1
  - Comment 2
    - Comment 4
- Comment 3

tildes/tildes/models/comment/comment_tree.py

@@ -92,6 +92,13 @@ class CommentTree:
                 comment.depth = 0
                 self.tree.append(comment)
 
+    @staticmethod
+    def _first_bottommost_reply(comment: Comment) -> Comment:
+        """Continuously gets the first reply from a comment until there is no more."""
+        while comment.replies:
+            comment = comment.replies[0]
+        return comment
+
     @staticmethod
     def _sort_tree(tree: list[Comment], sort: CommentTreeSortOption) -> list[Comment]:
         """Sort the tree by the desired ordering (recursively).
@@ -118,12 +125,13 @@ class CommentTree:
         elif sort == CommentTreeSortOption.RELEVANCE:
             tree = sorted(tree, key=lambda c: c.relevance_sorting_value, reverse=True)
         elif sort == CommentTreeSortOption.NEWEST_REPLY:
-            # sort by the creation time of the most recent reply in the entire tree, or
-            # the comment itself if it has no children. the comment's first reply is
-            # the newest reply since it has been sorted above
+            # sort by the creation time of the first bottom-most reply in the
+            # entire tree, or the comment itself if there are no replies.
+            # the comment's first bottom-most reply is the newest reply since
+            # it has been sorted above
             tree = sorted(
                 tree,
-                key=lambda c: (c.replies[0] if c.replies else c).created_time,
+                key=lambda c: CommentTree._first_bottommost_reply(c).created_time,
                 reverse=True,
             )