Enable identity verification for SAML signups

What does this MR do and why?

When signing up through an OmniAuth Provider, send a custom confirmation code instead of a Devise confirmation link and redirect to the identity verification page.

Screenshots or screen recordings

OmniAuth Provider	identity_verification disabled	identity_verification enabled
Group SAML with JIT* User Provisioning	identity_verification_disabled	identity_verification_enabled
Group SAML with SCIM User Provisioning	identity_verification_disabled	identity_verification_enabled
Google OAuth 2.0	identity_verification_disabled	identity_verification_enabled

*Using Just-In-Time (JIT) provisioning, user accounts are created when the user first signs in.

How to set up and validate locally

1. In rails console enable the identity_verification feature flag

   Feature.enable(:identity_verification)

2. Setup Group SAML with SCIM support for Okta and Google OAuth 2.0

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.