Switch to new rails credentials system

[davidsiaw]

Due to https://blog.engineyard.com/rails-encrypted-credentials-on-rails-5.2

The way our bootstrap script works may be obsolete. In the future we will need to generate the credentials.yml.enc file and store it on S3 to be downloaded by containers before running rails and set the RAILS_MASTER_KEY env var on all instances.

Or make rails credentials work with vault https://github.com/hashicorp/vault-rails