Redirect to customer portal or registration

What does this MR do?

Part of gitlab-org/growth/engineering#47 (closed)

Redirects users to customers app (old subscription flow) if they are not part of the experiment. If a user is not signed in, we redirect them to the registration screen where they later get redirected to the subscription page again by storing the URL.

It also adds the new experiment flag and enables the experiment for 10% of the users (which is part of our rollout strategy).

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

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