Make authenticated cookies lax

This protects our cookies a bit more. It makes sure that when a 3rdparty
websites embededs a public alendar for example. That all the users see
this in anonymous mode there.

It adds a small helper function.

In the future we can think about protecting other cookies like this as
well. But for now this is sufficient to not have the user logged in at
all when doing 3rdparty requests.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>