Support SAML Sign In

[davidje13]

e.g Okta:

• https://developer.okta.com/signup/
• https://developer.okta.com/docs/concepts/saml/
• https://developer.okta.com/docs/guides/build-sso-integration/integrate/

It should be possible to configure multiple SAML providers simultaneously

It may be desirable to provide a list of all retros within an organisation after logging in using saml

[davidje13]

Migrated to subproject