Scope and estimate adding user consent in social2SAML gateways

[alexstuart]

Also requires discussion whether this is required, since the social identity providers already have a consent page.

[alexstuart]

Documentation on how to enable consent module at https://simplesamlphp.org/docs/1.13/consent:consent and consent module now enabled on the Facebook gateway with cookie-based storage.

Plenty of discussion at Munich T5.4 meeting as to whether / what to put in such a gateway consent page.