Add RAVEN type update notice/circular; fixes #416

This implements a change introduced in ligo-followup-advocate!108 (merged) that adds a RAVEN type to our update circulars. This would be used if a coincidence comes in later than expected, say we have a significant GW detection but one of our targeted searches finds something later after the initial notice. Also this would be useful if we have a new sky map (say from Fermi-GBM officially) that we wish to give an updated combined sky map for.