If the request wasn't initiated by gitlab we shouldn't add the new
identity to the user, and instead show that we weren't able to link
the identity to the user.
This should fix: https://gitlab.com/gitlab-org/gitlab-ce/issues/56509
It consists of two parts:
1. Redirecting users to the configured external storage
1. Allowing the external storage to request the static object(s)
on behalf of the user by means of specific tokens
Part of https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6829
