Add alerting to Looker dashboards based on enrollments

[danielkberry]

It would be helpful if Looker could notify experiment owners when enrollments differ from expectations (either lower or higher than expected). Right now the API does not publish the expected enrollments, but if it did, those could be used to calculate differences (actual enrollments - expected enrollments) and send notifications based off of that.

┆Issue is synchronized with this Jira Task

[danielkberry]

It would also be helpful to have automated alerting when unenrollments differ between branches (fire off an alert when any of the warning tiles at the top show a warning)