Add Garmin LiveTrack session tracking

A Gmail-watching Apps Script (documented in docs/deployment.adoc) posts
the session ID from each LiveTrack email to a new webhook, which stores
it in data/livetrack.json; a new sidebar panel lists what's been
received.

public/js/map.js

@@ -7,6 +7,7 @@ import {
 import { RadarControl } from "/js/radar.js";
 import { GpxPanelControl } from "/js/gpx.js";
 import { GnssControl } from "/js/gnss.js";
+import { LiveTrackPanelControl } from "/js/livetrack.js";
 import { LAYERS_ICON } from "/js/icons.js";
 import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 
@@ -127,6 +128,7 @@ layerToggle.setActive("online");
 new GpxPanelControl(map).mount(sidebar);
 new RadarControl(map).mount(sidebar);
 new GnssControl(map).mount(sidebar);
+new LiveTrackPanelControl().mount(sidebar);
 
 function applyStyle(name) {
   layerToggle.setActive(name);