Add default terms opt in experiment

What does this MR do?

Issue: gitlab-org/growth/team-tasks#161 (closed)

As an experiment, we let users opt in to our terms and privacy policy by default, by stating that they confirm them by clicking "Register".

No changelog because behind the feature flag terms_opt_in_experiment_percentage.

How to test it locally

1. Enable Terms in http://localhost:3000/admin/application_settings/general
2. In the rails console, enable the experimentation feature flags

   Feature.enable_percentage_of_time(:signup_flow_experiment_percentage, 100)
   Feature.enable_percentage_of_time(:terms_opt_in_experiment_percentage, 100)

3. Open an incognito browser an go to http://localhost:3000/users/sign_up

Screenshots

Experiment disabled (current)	Experiment enabled

Snowplow events

1. On GET /users/sign_up: Growth::Acquisition::Experiment::TermsOptIn with action start and property control_group or experimental_group
2. On POST /users/sign_up: Growth::Acquisition::Experiment::TermsOptIn with action end and property control_group or experimental_group

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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