Support slot and prop for system-note-message and system-note-separator in NoteHeader component

Following https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8790#note_126286839:

The NoteHeader component has a default slot which is wrapped in .system-note-message and an actionText prop which is wrapped in .system-note-separator. We should

• support passing both as slots and props
• consider renaming the CSS classes because they don't always refer to system notes