Set cookie same site protection to nil

This is an attempt to fix an issue we're seeing where signing in with
Omniauth isn't working in Safari. Setting this value to `nil` (which
means the `SameSite` attribute is left unset) should in theory be the
same as setting to to `lax` (which means the `SameSet` attribute is set
to `Lax`), however it seems like Safari treats these differently:

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#lax

omniauth/omniauth-oauth2#155 (comment)

config/initializers/session_store.rb

@@ -1,3 +1,5 @@
+Rails.application.config.action_dispatch.cookies_same_site_protection = nil
+
 Rails.application.config.session_store :active_record_store,
                                        key: "_session_id",
                                        secure: Rails.env.production?,