app/models/concerns/thredded/post_common.rb
def mark_as_unread(user)
      if previous_post.nil?
        read_state = postable.user_read_states.find_by(user_id: user.id)
        read_state.destroy if read_state
      else
        postable.user_read_states.touch!(user.id, previous_post, overwrite_newer: true)
      end
    end
def mark_as_unread(user)
      if previous_post.nil?
        read_state = postable.user_read_states.find_by(user_id: user.id)
        read_state.destroy if read_state
      else
        postable.user_read_states.touch!(user.id, previous_post, overwrite_newer: true)
      end
    end
Marks all the posts from the given one as unread for the given user
 @param [Thredded.user_class] user