Update UI for AWS secret backend refactor
* Support empty AWS policy documents
* Try to make ARN input multiple
* move aws-role serializer to use the application serializer as the base
* support editing strings as JSON in the form field component
* update model, form and show to use form-component component, and swap fields based on credential type
* fix tests
* unify credential generation for aws and remove the STS specific action in the UI
* add label to the new json string form field
Matthew Irish