Deploy Atlantis to automate Terraform workflow

[d3adb5]

While personally not a fan of Atlantis, it lets us keep credentials away from CI/CD pipelines. Since we want to keep secrets in HashiCorp Vault, it makes sense to have something like Atlantis on the cluster instead of adding GitHub Actions secrets that are too close to public repositories for comfort.

This issue is mainly for deploying Atlantis itself, not necessarily setting up credentials, which may be tracked through #13.

We may go back on this, of course. Nothing is set in stone.

[d3adb5]

Considering we have no plans to create private repositories right now, and the mitigations for running Atlantis against public repositories eliminate some of its advantages, this won't be done.