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.

lib/maptool/livetrack_store.rb

@@ -0,0 +1,63 @@
+require "json"
+require "fileutils"
+
+module Maptool
+  # JSON-file-backed store of Garmin LiveTrack session IDs, appended to by a
+  # webhook whenever a new LiveTrack email arrives (see docs/deployment.adoc).
+  # Validity/expiry of a session isn't tracked here — checking whether a
+  # given ID still has live data is up to whoever polls Garmin with it.
+  class LiveTrackStore
+    def initialize(path)
+      @path = path
+      @mutex = Mutex.new
+      FileUtils.mkdir_p(File.dirname(@path))
+      File.write(@path, "[]") unless File.exist?(@path)
+    end
+
+    # Records `session_id`, moving it to the front if already present.
+    def add(session_id)
+      @mutex.synchronize do
+        sessions = read
+        sessions.reject! { |s| s["id"] == session_id }
+        sessions.unshift({ "id" => session_id, "received_at" => Time.now.utc.iso8601 })
+        File.write(@path, JSON.pretty_generate(sessions))
+      end
+    end
+
+    # Most recently received first.
+    def list
+      read
+    end
+
+    private
+
+    def read
+      JSON.parse(File.read(@path))
+    rescue Errno::ENOENT, JSON::ParserError
+      []
+    end
+  end
+end
+
+if __FILE__ == $0
+  require "tmpdir"
+
+  Dir.mktmpdir do |dir|
+    store = Maptool::LiveTrackStore.new(File.join(dir, "livetrack.json"))
+    raise "expected empty list" unless store.list == []
+
+    store.add("abc123")
+    store.add("def456")
+    raise "expected 2 sessions" unless store.list.size == 2
+    raise "expected newest first" unless store.list.first["id"] == "def456"
+
+    store.add("abc123")
+    raise "expected re-added session moved to front, not duplicated" \
+      unless store.list.map { |s| s["id"] } == %w[abc123 def456]
+
+    reloaded = Maptool::LiveTrackStore.new(File.join(dir, "livetrack.json"))
+    raise "expected data to survive reload" unless reloaded.list.size == 2
+  end
+
+  puts "ok"
+end