Draw aids to navigation as buoys

A buoy, beacon, RACON, light or IALA mark in the vessel feed was drawn
as a ship. A vessel record whose type names one, or whose MMSI is an
aid-to-navigation station's (99MIDxxxx), is now isAidToNavigation and
vesselModelSpec draws it as the fleet set's buoy of its mark (cardinal
N/E/S/W, port or starboard hand, safe water, isolated danger, special
mark; a red can when the type names no mark), at its built size and
flagged fixed so its model does not turn to a heading. Its icon is a
disc instead of a chevron.

AISStream now subscribes to AidsToNavigationReport by default, and the
server names each report's aid type code (0 to 31) as its kind instead
of reading it as a ship type.

server/providers/vessels/ais-live.js

@@ -27,6 +27,8 @@ const AISSTREAM_DEFAULT_MESSAGE_TYPES = [
   'ExtendedClassBPositionReport',
   'ShipStaticData',
   'StaticDataReport',
+  // Buoys, beacons and other aids to navigation (AIS message 21).
+  'AidsToNavigationReport',
 ];
 // Watchdog budgets (policy lives in src/data/aisWatchdog.js). Silence is
 // REPORTED quickly and ACTED ON slowly: a dead feed must read as dead within