Adjust experimentation group without MRs

Parent issue: #207538 (moved)

Background

The Growth team is currently using a Experimentation class which is using feature flags to turn experiments on and off. When an experiment is on, we bucket users with a cookie to be part of one of the groups (experiment or control group).

Issue

Currently, the ratio of experiment and control group is hard coded with an enabled_ratio. This was fine in the beginning but with more experiments in the future, we don't want to open a merge request every time to change ratio.

Goals

1. Adjust the experiment ratio with chatops in the future, by running /chatops feature set new_growth_experiment 50 in Slack.
2. A user must have a consistent experience. They must not switch between experiment and control group during the same session.
3. We might want to share an experiment group between .com and customers app. The customers app is already using Unleash.