Set session cookie `SameSite` attribute to `Lax` for main site (#11721)
We are now injecting ads via addons,
which doesn't make use of the sustainability endpoint.
That endpoint was used to check if the user was a Gold member,
and not show ads to them.
The usage of the sustainability API was he only reason to set the
`SameSite` attribute to `None`.
Using Lax is more secure, as browser will never send the cookie
in a cross-site request.