Improve steps to download metadata XML

[juliusknorr]

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