Record terms opt in experiment users

What does this MR do?

We saw that the experiment (no checkbox to accept terms) performed 7% worse than the control group, see: gitlab-org/growth/team-tasks#179 (comment 398622570). However, we want to ensure that the data is correct.

To have a better data quality, we want to also know which users are part of the experiment. This should help us to filter out bots from automatic sign up scripts.

We now only track the end of the experiment phase only for valid sign up data.

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