Use optional or default 301 redirect instead of 302 redirect for non-logged in users

Related to https://gitlab.com/gitlab-com/infrastructure/issues/4782

It was brought up that this should be a 301 instead of a 302. Bringing up here in case this is something that we think self-managed customers would want as well. I think for gitlab.com it is likely we will stop using this feature and instead redirect in haproxy.