Redirect to /identity_verification/success URL for tracking

What does this MR do and why?

• Follows from !130799 (merged) and resolves issues https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/477
• In the previous MR, we had removed the intermediary success page, which caused problems with tracking a user in the registration funnel.
• In this MR, we have added the intermediary success page back.

Screenshots or screen recordings

Before	After
before	after

How to set up and validate locally

1. Turn on feature flag

Feature.enable(:arkose_labs_signup_challenge)
Feature.enable(:identity_verification)

2. Update application setting to turn on verification

ApplicationSetting.first.update(require_admin_approval_after_user_signup: false, email_confirmation_setting: "hard")

3. Get credentials from 1Password to connect to our external services

ApplicationSetting.first.update(arkose_labs_public_api_key: XX, arkose_labs_private_api_key: XX)

4. Sign-up as a new user from /users/sign_up. You should see a page with content Help us keep GitLab secure and you will be asked to verify your email address.
5. You can find the verification code on http://127.0.0.1:3000/rails/letter_opener.
6. Once verified, you should see the success page. You should get automatically redirected to the dashboard.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.