Migrates RoundRobin rollout strategy into the invite_email_experiment

What does this MR do?

It moves some of the logic that's in ApplicationExperiment into the Members::InviteEmailExperiment to keep that experiments behavior consistent throughout its run.

This code has been migrated to the gem and is available for us to use, but just for consistency in experiment data we're not going to use it on the Members::InviteEmailExperiment experiment while it's running.

However, we can now migrate the concepts out of ApplicationExperiment and in so doing, reduce some of the complexity around that inheritance.

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because refactor/cleanup.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

The risk is that we could break something about the Members::InviteEmailExperiment data collection and variant assignment. I've done my best to ensure this isn't a problem and have reviewed that this experiment remains working in development.