Dockerize KeyConjurer

[punmechanic]

This pull request previews changes that would be made if we converted all of KeyConjurer's server elements to produce Docker artifacts rather than relying on out-of-band Terraform configuration.

• Vault is no longer used within Go; instead, we use the Hashicorp Vault lambda extension which retrieves secrets for us. This removes a dependency and simplifies the initialization code.
• Instead of having to maintain a separate S3 bucket and Cloudfront distribution, we can host a container on ECS for the frontend and have a load balancer direct requests to it.