Draft: Use new ExperimentSubject model with experimentation helper methods

What does this MR do?

Makes it much easier to make use of the ExperimentSubject model as part of the overall experimentation process.

I don’t think this change needs a changelog as it is a fully developer-facing change.

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

Other Todos

• Monitor !50679 (merged) and either notify @alexbuijs of this change if this MR merges first or use the changes in this MR to update !50679 (merged) if his MR merges first.
  • !50679 (merged) merged first

Related to #289941 (closed)