Add support for charts with multiple series to SLI alerts

With our MVC of SLI alerts, we only support charts with a single series. We should add the option to support charts with multiple series. It could expose a dropdown listing the current series, and letting the user pick which ones to alert on.

This way if you have a chart showing the response times for p50, p90, and p99 you could choose to set an alert for only one of these, or separate alerts for each series.