Forwarding to IDPs based on email domain of user

[Nate Klingenstein]

> This is one of the ways wsignin is better than SAML.

The reason it's not present in SAML is that we thought privacy was important on the Internet back then, and we didn't want to ask users to present their (unverified) identity to the service just so they could start a login that may have resulted purely in non-personally identifying information.  The world turned out to not care about privacy much since you can monetize said information so well.