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.

.gitea/workflows/ci.yml

@@ -82,6 +82,7 @@ jobs:
           GITEA_OAUTH_CLIENT_ID=${{ secrets.OAUTH_CLIENT_ID }}
           GITEA_OAUTH_CLIENT_SECRET=${{ secrets.OAUTH_CLIENT_SECRET }}
           MAPTOOL_ALLOWED_GITEA_USER=${{ secrets.MAPTOOL_ALLOWED_GITEA_USER }}
+          LIVETRACK_WEBHOOK_SECRET=${{ secrets.LIVETRACK_WEBHOOK_SECRET }}
           EOF
           chmod 600 /srv/docker/maptool/.env