Create GitHub Actions job to monitor metadata endpoint

[thekaveman]

This will be a follow-up to #510

Once the /metadata endpoint is live in production, let's add a GitHub Actions workflow:

• Runs early in the morning e.g. 6am Pacific
• For each agency
• Hit their /metadata endpoint and get the JSON back
• Check that the db.timestamp is the same date as the current date
• Fail the workflow if not, notify Slack