Add page for new Identity Verification flow

Implements Implement new identity verification flow show action task of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/629.

Context

Currently users go through identity verification as part of the signup process. Moving forward, we want to allow users that have already started using Gitlab to go through identity verification to complete verification methods–phone number and/or credit card verification–that they didn't go through during signup in exchange for in-app benefits (e.g. more CI minutes, etc.) (see epic).

What does this MR do and why?

This MR adds a new controller and view for the new Identity Verification flow for active (signed in at least once) users. The new page behaves almost similar to Identity Verification during signup with the following differences:

1. It requires a signed in user
2. It only shows up to two identity verification methods–phone and/or credit card. This is because users visiting this page are already active users (last_sign_in_at is present and are email-verified).

What is NOT in this MR

To make review easier, this MR does not include some tests (see notes in diffs) and other controller actions required to make the new Identity Verification flow fully functional. Follow-up MRs will introduce the missing pieces and all new behavior are hidden behind opt_in_identity_verification WIP feature flag.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Enable the relevant feature flags

   $ rails c
   > Feature.enable(:opt_in_identity_verification)
   > Feature.enable(:identity_verification_phone_number)

2. Login with root

3. Visit /-/identity_verification

4. Verify that the new Identity Verification page is displayed with phone number verification step