See https://github.com/simplesamlphp/simplesamlphp-module-casserver

Fixes #873

SHA-1 is still supported but needs to be configured explicitly if you need it.

Per suggestion from @tvdijen

Add some basic documentation on how the new Twig templating system works.

It was already added under 1.15

Per https://groups.google.com/d/msg/simplesamlphp/ATKQksysR5s/3VhjdLbTAgAJ
there is a feature to allow IdPs to respond to requests sent to different
IdP entityIDs.