travel-advice-publisher: Error Monitoring for Travel Advice Alerts

Confirming email delivery
- a job runs every 15 mins in email-alert-api
- if it's an hour since a travel advice alert was created and no emails have been delivered for that alert, the 2nd line/on call team are paged.
- More details: https://github.com/alphagov/email-alert-api/blob/main/docs/alert_check_scheduled_jobs.md
Application error monitoring in Sentry
A healthcheck in email-alert-api
- We use GOV.UK Notify to deliver emails
- email-alert-api is the app which communicates with GOV.UK Notify
- We have a healthcheck set up to ensure that email-alert-api can connect to GOV.UK Notify
- If the healthcheck fails, an urgent alert is shown in Icinga