Extract DiscussionNotes component from NoteableDiscussion

This is part of https://gitlab.com/gitlab-org/gitlab-ce/issues/55566.

Extract this bit https://gitlab.com/gitlab-org/gitlab-ce/blob/a5174cf0fcfd0ae77d25ac5208b7aa371d7d7b73/app/assets/javascripts/notes/components/noteable_discussion.vue#L422-475 into a new DiscussionNotes (or DiscussionComments) component.