Add paid signup tracking events

What does this MR do?

Part of #208802 (closed) to improve events for the new paid signup flow.

Two things to be aware of:

1. Users who go to /-/subscriptions/new who are not signed in, get redirected to /users/sign_up. After they successfully created an account or signed in, we redirect them to /-/subscriptions/new again. However, when they go to -/subscriptions/new, we would track the experiment_start event a second time. Therefore, we added the param experiment_started to not call track them twice
2. For the sign_up_view event, we only want to track users who got redirected from /-/subscriptions/new, therefore we have added the redirected_from=checkout param.

Screenshots

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