Add Clear-Site-Data header

This adds a Clear-Site-Data header to the logout response which will delete all relevant data in the caches which may contain potentially sensitive content.

See https://w3c.github.io/webappsec-clear-site-data/#header for the definition of the types.

Ref https://twitter.com/mikewest/status/877149667909406723



Signed-off-by: Lukas Reschke <lukas@statuscode.ch>