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/icons.js

@@ -47,3 +47,9 @@ export const WAYPOINT_ICON = `
 </svg>`;
 
 export const TRACK_ICON = GPX_ICON;
+
+export const LIVETRACK_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M12 21s7-7.58 7-12a7 7 0 0 0-14 0c0 4.42 7 12 7 12z"/>
+  <path d="M8.5 9a3.5 3.5 0 0 1 7 0" stroke-dasharray="1.5 2.2"/>
+</svg>`;