Add better uptime check

[chrismclarke]

External uptime checks simply look for response code (200), so only testing firebase hosting service. More useful would be a custom cloud function (e.g. /healthcheck or /uptime) that queries a couple URLs (home and re-routed page), and checks that expected content appears (e.g. header elements). This would handle case where broken javascript is uploaded or a mismatch between server API files and content (from SEOHost function).