[P1] Run Terraform in CI

[willroberts]

Running Terraform in CI would solve a few things:

• Avoiding version mismatch issues on local machines
• Allowing anyone to work on the infra without distributing AWS access keys
• Providing a way to notify us when the expected state differs from the actual state

Something like https://www.runatlantis.io/ would be good here, to avoid building something similar ourselves.