feat(cookies): Use the same-site attribute to lax (#8626)

CSFR-prevention is already implemented but adding this doesn't hurt.

A brief introduction to Same-Site cookies (and the difference between strict and
lax) can be found at
https://blog.mozilla.org/security/2018/04/24/same-site-cookies-in-firefox-60/

TLDR: We use lax since we want the cookies to be sent when the user navigates
safely from an external site.

spec/rails_helper.rb

@@ -29,6 +29,7 @@ Devise::Test::ControllerHelpers.module_eval do
       value: resource.activate_session(warden.request),
       expires: 1.year.from_now,
       httponly: true,
+      same_site: :lax,
     }
   end
 end