enhance: set longer expiry time for `isAdmin` cookie
enhance: set longer expiry time for `isAdmin` cookie

Set to 10 years now. Also let's see if the `SameSite` flag fixes the issue with it being set in an iframe.

owid/owid-grapher,owid/owid-grapher,owid/owid-grapher,owid/owid-grapher,owid/owid-grapher