Experiment: New user onboarding default to viewing issue board

What does this MR do?

This MR creates an experiment wherein we send a portion of new user signups straight to the “Learn GitLab” project’s onboarding issues board directly after the final step of the registration process rather than taking them to the group landing page and guiding them to the project & issue board via popovers.

See the issue for more details.

No changelog or documentation changes are needed at this time as this is an experiment behind a feature flag.

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

Addresses #246795 (closed)