Support for client-side A/B testing

[sammacbeth]

Reviewer:

Description:

• Handle the cohort option in the remote config, by randomly selecting a cohort using the provided cohort weights. (Implementation doc: https://app.asana.com/0/1208889145294658/1208680059799038/f).
• Provide an API for sub-features, upon seeing that a cohort was assigned, can trigger enrollment, which sends a pixel (https://app.asana.com/0/1208889145294658/1208747415972722/f).
• Adds a browser UI to allow rollouts and cohorts to be manually adjusted for testing purposes.

Steps to test this PR:

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications

[github-actions]

Privacy Review task: https://app.asana.com/0/69071770703008/1209161143946992