Add comment in message for external sky map `.fits` and `.png` that indicate which type of alert created from

As pointed out in https://gracedb.ligo.org/events/E415314/view/, it was not clear to non-experts which GRB sky map corresponded to which notice type. We can easily add that here and here by using the given notice_type and a dictionary that maps the notice type to the text name we want to add to the messages.