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.

views/index.erb

@@ -49,3 +49,11 @@
 
   <div class="gpx-tree"></div>
 </div>
+
+<div id="livetrack-panel" class="basemap-panel" hidden>
+  <div class="basemap-panel__header">
+    <strong>LiveTrack sessions</strong>
+    <button type="button" class="basemap-panel__close" aria-label="Close">&times;</button>
+  </div>
+  <ul class="livetrack-panel__list"></ul>
+</div>