Experiment - Customize Registration page for invites [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

• Creates an experiment on accepting email invites by which:
  • users in our candidate experience will redirect to a special registration page for invites with customize UI text and devoid of exit CTA's
  • tracks the viewing and successful registration of control(users not in our new experience) vs candidates(users in our new experience)
• Remediates the wording on the Trial registration page so that the By clicking text matches the button text of Continue.

How to test locally(GDK)

1. Enable the experience(only one of these should be picked at a time):
   • candidate(new experience): bundle exec rails runner "Feature.enable(:invite_signup_page_interaction)"
   • original experience: bundle exec rails runner "Feature.disable(:invite_signup_page_interaction)"
2. Invite a user by email to a project or group(ensure rails-background-jobs is running in gdk).
3. Open letter opener
4. Click Join now and open in an incognito window

Screenshots (strongly suggested)

control(current)	candidate(new experience)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because it is behind a feature/experiment flag.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #327239