Improve DB alarm to indicate that the DB is down

[jzbahrai]

Description

As a user, I want to know if the DB is down. Our alarms should go off if the DB is down, not just when there are celery errors. This is a followup from this incident: 2023-02-21-celery-errors-database-upgraded-automatically, where the devs discovered the DB was down because of celery errors (and wouldn't have otherwise).

Acceptance Criteria

• [ ] If the DB goes down, we should get a cloudwatch alarm indicating likewise.

QA Steps

• [ ] Test that the alarm goes off when the DB is down

[ben851]

I've investigated this a bit and it seems as thought we're already subscribed to aws rds events for failures etc.

I'm going to investigate whether there are some secondary metrics we can use to infer an issue.