Add tracking for Email Verification

What does this MR do and why?

This adds backend Snowplow tracking for email verification, part of barriers to entry.

The following events are tracked:

1. an invalid code is entered

   se_category: "IdentityVerification::Email",
   se_action: "failed_attempt",
   se_property: "invalid",

2. the code entered has expired

   se_category: "IdentityVerification::Email",
   se_action: "failed_attempt",
   se_property: "expired",

3. the attempt was rate limited

   se_category: "IdentityVerification::Email",
   se_action: "failed_attempt",
   se_property: "rate_limited",

4. a new code has been sent

   se_category: "IdentityVerification::Email",
   se_action: "sent_instructions",

5. the attempt was successful

   se_category: "IdentityVerification::Email",
   se_action: "success",

Issue: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/123

How to set up and validate locally

1. Install Snowplow micro

   gdk config set snowplow_micro.enabled true
   gdk reconfigure

2. In rails console enable identity verification

   Feature.enable(:identity_verification)
   ApplicationSetting.first.update(require_admin_approval_after_user_signup: false)
   ApplicationSetting.first.update(send_user_confirmation_email: true)

3. Visit http://localhost:3000, logout and create a new user
4. Visit http://localhost:3000/rails/letter_opener and copy the code from the email
5. Fill in the code on the code verification page
6. Visit http://localhost:9091/micro/good

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.