Fix opt in email option during registration

What does this MR do and why?

• adds opt in email option in certain situations and defaults it to true for others.
  • reverts only parts of !130739 (merged) as this is technically a new feature, but considered a bug as we thought this was working one way and wasn't at all.
• this allows marketing/Iterable teams to correctly by sending those values to CustomersDot in the lead creation process see functionality added in https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/8266 to remove hardcoding and allow our sends here to be observed. opt in or out users from marketing emails

Screenshots or screen recordings

Before	After
Screen_Recording_2023-09-15_at_2.48.08_PM	Screen_Recording_2023-09-15_at_2.43.21_PM

How to set up and validate locally

1. Ensure GDK is setup to simulate SaaS
2. Enable check_namespace_plan in rails console
   • ApplicationSetting.first.update(check_namespace_plan: true)
3. Restart GDK
4. Register as a new user. You can confirm email, if needed, by using the value from Rails.application.routes.url_helpers.user_confirmation_path(confirmation_token: User.last.confirmation_token) in rails console
5. After registration, and any confirmation steps, you should land on the welcome page(in onboarding).
6. Toggle the email check box and the Who will be using GitLab? choice.

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.

Related to #425177