New sort method for comments: by newest leaf, similar to "activity"

A new sort method for comments could account for replies to existing top-level comment threads, not just when the top-level comments were posted.

Related thread: https://tildes.net/~tildes/1k7/sort_comments_by_newest_leaf_not_branch

Similar suggestion: https://tildes.net/~tildes/272/simple_suggestion_two_different_types_of_new_sort_for_comments