Redirect instead of returning 404 when verified

What does this MR do and why?

This MR does 2 things to prevent 404's when showing the identity verification page:

1. In the RegistrationsController, set the verification_user_id in the session just before redirecting to the identity_verification_path, this reduces the chance of being redirected without the session being set.
2. Instead of rendering a 404 when the verification_user_id was not present in the session or when the user was already verified, now redirect to the root if the session was not set and redirect to the success_identity_verification_path when the user is verified.

Closes https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/232+

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