New Check: Unexpected Unenrollments

[jaredlockhart]

This week we saw a case where a recipe was unenrolling at an unexpectedly high rate early in the experiment. Data science noticed it when looking at the grafana dashboard, but it would be great if we could build some kind of automated check for this case.

https://grafana.telemetry.mozilla.org/d/XspgvdxZz/experiment-enrollment?orgId=1&var-experiment_id=pref-activity-stream-chrome-switchers-card-for-onboard-release-73-75-bug-1621101&from=1583712000000&to=