Show alert thresholds on dashboard

As a follow up from #5158 (closed), we should show the alert thresholds on the dashboard.

• This could take the form of a simple red line for threshold based alerts, and is displayed when the series is near it.
  • Ideally, this could also have a "filled in" appearance, with the direction of the alert. For example if the alerting rule was error rate > 1%, then you would see a red line at 1% and then a slightly red shaded portion above it. Similarly, if it was something like Availability < 99%, you would see a red line at 99% and then a shaded portion below it.

Design

The alert threshold would not be visible on the graph unless the value was near it. So it would either not be rendered at all, or just be allowed to overflow off the top.

• Background color: #$red-050
• The bottom border: #$red-400
• The middle border: $red-200

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