Implement first phase of duo enterprise in-app trials

What does this MR do and why?

Implement first phase of duo enterprise in-app trials. A lot of code used here is copied from duo pro implementation. A lot of work was done to try and DRY up certain areas before this MR in efforts to make the smallest vertical change and can be observed in the execution details of the issue.

• solve for the case of only 1 eligible namespace.
• guard with feature flag to control rollout.
• updates lead forms everywhere with preferred wording since the constants used are shared
  • hand raise lead
  • user registration for trials
  • in-app ultimate trials, duo pro trials and duo enterprise
  • see https://gitlab.com/gitlab-org/gitlab/-/issues/468050#note_2023216682
• updates the in-app trials legal/terms as the constants used are shared
  • in-app ultimate trials, duo pro trials and duo enterprise
  • see https://gitlab.com/gitlab-org/gitlab/-/issues/468050#note_2023216682
• updates the in-app trials button width on all to be auto as the code used is shared
  • in-app ultimate trials, duo pro trials and duo enterprise
• see https://gitlab.com/gitlab-org/gitlab/-/issues/468050

What parts of #468050 doesn't this do in order to scope this smaller

• tracking at the controller layer
• benefits/achievement part of UI
• success flash message

These will be covered in a subsequent MR

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

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Screen	Before	After
duo enterprise trial	N/A
duo pro trial
ultimate trial
hand raise lead
form on user registration

How to set up and validate locally

1. Setup GDK to simulate SaaS.
2. Enable duo_enterprise_trials feature flag in bin/rails console with Feature.enable(:duo_enterprise_trials).
3. Seed the add-ons for your instance from the terminal if they haven't been created yet(rerun will not harm anything if you do have them)
   • FILTER=add_ons bundle exec rake db:seed_fu

In-app Duo Enterprise Trials(main goal of this MR)

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1. Create new group.
2. In rails console make it ultimate.

bin/rails c

Group.last.gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'ultimate'))

3. Visit /-/trials/duo_enterprise/new?namespace_id=<group id created>.
4. See form. The submission is covered by feature specs delivered in this MR that aren't reproducible unless CustomersDot is setup locally.

In-app Duo Pro Trials(form text changes)

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1. Create new group.
2. In rails console make it premium.

bin/rails c

Group.last.gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'premium'))

3. For lead, visit /-/trials/duo_pro/new.
4. For trial, visit /-/trials/duo_pro/new?step=trial.
5. Notice text changes to inputs and terms.

In-app Ultimate Trials(form text changes)

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1. Create a new group
2. Visit /-/trials/new.
3. Notice text changes to inputs and terms.

Hand raise lead(form text changes)

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1. Navigate to premium group created above and go to Settings -> Billings
2. Click on 'Contact sales.

User registration for trials(form text changes)

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1. Ensure sign ups are enabled and email confirmation set off.
   • Make sure Admin > Settings > General > Sign Up Restrictions match the below (no admin approval, no email confirmation)
2. Sign up for non SSO(fill in user name and password) with trial(with /-/trial_registrations/new path).
3. Fill in the welcome page and submit.
4. Observe the changes in text on the next form as noted in description.

Related to #468050