SPNEGO and _eventId_authn/SPNEGO button

[Cantor, Scott]

> I'm not sure actually sure what it's doing under the covers
> now and I do want to take a look at it but at best I'd want it
> failing in a more controlled fashion.

I reviewed this a little. There's either a hook for doing this that isn't working or I may have just forgotten to pull the final vestiges of the original password "extended flows" feature that allowed the button to behave that way, but I won't know which until I can review the commit history more.

If it should work, I'll fix it and if it shouldn't, I'll remove the broken hook for it. Filed as IDP-2348.

-- Scott