Verify referer before adding to session

What does this MR do and why?

Fixes https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/725

When performing identity verification we save the referer in the session so we can re-direct the user back to where they came from once identity verification is complete. There are some scenarios where the referer could be the identity verification page which would result in the user ultimately getting re-directed to the root path upon completion of identity verification. This MR adds a check to ensure we only set the session variable if the referer is not the identity verification path.

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