Move email opt in to first step in registration for SaaS

What does this MR do and why?

• Move the email opt in checkbox from welcome page to user registration page
• default checked to true
• ensure omniauth submission honors the current checkbox setting
• see https://gitlab.com/gitlab-org/gitlab/-/issues/482911 for the reasoning.
• see https://gitlab.com/groups/gitlab-org/-/epics/15076#design-proposal for design and discussions to see how we got to the current design.

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

note: see text update for checkbox in !164411 (comment 2153433216)

Click to expand

page	Before	After
user registration
trial user registration
trial user registration toggling omniauth form param change	n/a	Screen_Recording_2024-10-10_at_3.07.06_PM
welcome page
welcome page toggling	Screen_Recording_2024-10-10_at_3.03.47_PM	Screen_Recording_2024-10-10_at_3.04.46_PM

How to set up and validate locally

note feature specs in ee/spec/features/registrations/saas/ cover onboarding along with the feature specs added in this MR.

Click to expand

1. Setup to simulate SaaS and restart GDK
   • Make sure Admin > Settings > General > Sign Up Restrictions match the below (no admin approval, but hard email confirmation)
2. Sign out if necessary.
3. Enable feature flag to match screenshots above if desired, using the command Feature.enable(:duo_enterprise_trials_registration) in the rails console.
4. Navigation to the /-/trial_registrations/new and /users/sign_up
5. Notice the changes shown in the screenshots on registration pages.
6. Fill out form and submit.
7. Confirm email by finding the confirmation path from last User created in rails console
   • Rails.application.routes.url_helpers.user_confirmation_path(confirmation_token: User.last.confirmation_token)
8. You should be on the sign in page, log back in.
9. We should now see the welcome page and notice the opt in is not there, even when you click 'Just me'

Related to #482911