Create new ExperimentSubject model

What does this MR do?

Creates a new experiment_subjects table & model to take the place of the existing experiment_users table & model.

This new model & table enable us to record different subject types for different experiments. Currently, we can only record experiment_users, so if we run an experiment on a namespace or project, we don’t have a clear way of how to report that.

A related change that actually enables us to pivot experiments on different subjects using experimentation.rb has recently been merged into master and a product-initiative issue is in development which will pivot its experiment on a Group.

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

Related to #250800 (closed)