Improve steps to download metadata XML

It is quite odd how one can setup SAML by generating a metadata XMl file for the IdP. When checking the following fields seemed required:

• Service Provider Data: X509 certificate of the service provider
  • Hidden by default
  • We can hint to use openssl to generate one or even provide a button to generate one
• Service Provider Data: private key of the service provider
  • Hidden by default
  • We can hint to use openssl to generate one or even provide a button to generate one
• Identity provider Data: Identifier of the IdP entity (must be a URI)
• Identity provider Data: URL Target of the IdP where the SP will send the Authentication Request Message
• Identity provider Data: Public X.509 certificate of the IdP
  • Hidden by default

Additionally we should:

• Properly indicate that this is a missing field when saving instead of just showing Invalid metadata and hiding an error message in the response

Start by locating the SAML metadata XML setup form and the save path that currently reports only invalid metadata. Review how the listed service-provider and identity-provider fields are displayed and validated. Done means required fields are clearly indicated, hidden certificate and key fields have generation guidance or controls, and missing-field errors are visible when saving.