SAML metadata for our SP

[humphd]

What would you like to be added:

We need to provide Seneca with the SAML metadata for our app. We can use generateServiceProviderMetadata() in passport-saml to do this, and even provide it as an endpoint in the app

Why would you like this to be added:

I spoke with ITS about connecting our staging server to their staging SSO box, and in order to do it, they need our "Service Provider Metadata" (SP). To do this we'll have to have our SSL and certs all setup first.

Depends on #640, #634.