added Comment#last_activity_at and ensured CommentThread#last_activity_at takes that into account

Author: Oaphi

app/models/comment.rb

@@ -19,6 +19,12 @@ class Comment < ApplicationRecord
 
   validate :content_length
 
+  # Gets last activity date and time on the comment
+  # @return [DateTime] last activity date and time
+  def last_activity_at
+    [created_at, updated_at].compact.max
+  end
+
   def root
     # If parent_question is nil, the comment is already on a question, so we can just return post.
     parent_question || post

app/models/comment_thread.rb

@@ -52,7 +52,8 @@ def can_access?(user)
   # Gets last activity date and time on the thread
   # @return [DateTime] last activity date and time
   def last_activity_at
-    [created_at, locked_at, updated_at].compact.max
+    last_comment_activity_at = comments.map(&:last_activity_at).max
+    [created_at, locked_at, updated_at, last_comment_activity_at].compact.max
   end
 
   # Gets a list of user IDs who should be pingable in the thread.