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.

.gitignore

@@ -13,6 +13,9 @@
 /data/gpx/*
 !/data/gpx/.gitkeep
 
+# Garmin LiveTrack session IDs received via webhook
+/data/livetrack.json
+
 # Editor / OS
 .DS_Store
 *.swp