Add circle/slack notifications

[jaredlockhart]

We can add a slack integration to our circleci conf that will notify in slack when a main branch build/deploy fails. Right now those build/deploys can fail after a merge to main and we don't receive any notification, so deploys can seize and stop until someone happens to notice.

Docs: https://circleci.com/blog/circleci-slack-integration/

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