Don't expose AWS secrets in ES administration

Summary

This is a follow-up to !17230 (closed) / !18254 (merged) / !20421 (merged).

The ES administration displays the AWS secret access key, though it's initially shown with asterisks and needs to be exposed with a click.

Improvements

We need to mirror the changes to the old admin UI from f09f7b53.

In the backend, we should not return aws_secret_access_key in the API endpoints, but return an indication if it is set or not. When updating an index, an empty value for the attribute should be ignored and not overwrite an existing value.

In the frontend, we should display the label Enter new AWS Secret Access Key on the form field if the attribute is set.

Involved components

• ee/app/assets/javascripts/elasticsearch/
• ee/lib/api/elasticsearch_indices.rb