Extract ReplyPlaceholder component from NoteableDiscussion

This is part of #55566 (moved).

Extract this bit https://gitlab.com/gitlab-org/gitlab-ce/blob/8b4b7caefa7fba49a8b199edfde8315a4c897a68/app/assets/javascripts/notes/components/noteable_discussion.vue#L431-438 into a new ReplyPlaceholder component.