Add credentials using a generic EncryptedConfiguration class by dhh

The combination of config/secrets.yml, config/secrets.yml.enc, and SECRET_BASE_KEY is confusing. It's not clear what you should be putting in these secrets and whether the SECRET_BASE_KEY is related to the setup in general.This PR will deprecate secrets.yml* and instead adopt config/credentials.yml.enc to signify what these secrets are specifically for: Keeping API keys, database passwords, and any other integration credentials in one place.