Secure storage of Elasticsearch credentials

[techgique]

Addition of Elasticsearch basic auth headers in #210, but uses plaintext storage of credentials. We'll want to move to an encrypted storage of these but without Rails's credentials and master key here, I'm not sure how we'll handle it. Not sure if we can include some of Rails's gems like ActiveSupport to grab those features but that'd be nice to keep all of our Ruby code handling credentials similarly.