Add Okta/SAML support

[mbland]

Since OKTA has become a common Single Sign-On solution for many businesses, we should add it as an auth provider via Passport.

Some helpful resources:

• https://developer.okta.com/standards/SAML/setting_up_a_saml_application_in_okta
• https://www.npmjs.com/package/passport-saml
• https://github.com/ThoughtWorksInc/okta-samples/tree/master/okta-node-express
• https://dzone.com/articles/integrating-nodejs-ruby-and
• https://www.okta.com/blog/2016/12/what-is-saml/
• https://auth0.com/blog/how-saml-authentication-works/
• https://developers.onelogin.com/saml
• https://en.wikipedia.org/wiki/Security_Assertion_Markup_Language
• https://en.wikipedia.org/wiki/SAML_2.0

[mbland]

Actually, @akashvbabu discovered that Okta spoke OpenID Connect and implemented it in #150.