Create identity_verification_enabled? and identity_verified? methods

Context

Currently, users go through identity verification (let's call this signup IV) as part of the signup process. Moving forward, we want to require users¹ that have already started using Gitlab to go through the same identity verification process (let's call this active user IV) where they can verify a phone number/credit card in exchange for in-app benefits (e.g. ability to run pipelines, create more groups, etc.).

See https://gitlab.com/groups/gitlab-org/modelops/anti-abuse/-/epics/32+ for more info.

¹ Limited to users that did not go through phone number and/or credit card verification during signup.

What does this MR do and why?

In this MR, we rename identity_verification_enabled? and identity_verified? that are used for signup IV flow to signup_identity_verification_enabled? and signup_identity_verified?, respectively.

We then introduce new methods using the old names that will be used for active user IV flow.

MR acceptance checklist

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