Add Experiment Subjects

What does this MR do?

1. It adds an optional subject to experiments, which will be used to determine the group (experiment or control) for the experiment
2. The subject will also be used when tracking experiments. We use an MD5 hash of the subject to not expose any possible user ids to Snowplow
3. If no subject is given, we fallback to cookies (which is how current experiments behave)
4. It adjusts the API of experiments a bit
   • enabled? becomes active? since it checked if the experiment is even started
   • enabled_for_attribute becomes in_experiment_group to make it clearer that this is for a given subject

Screenshots (strongly suggested)

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