Moving from https://github.com/catapult-project/catapult/issues/2270

Currently, ignoring alerts assigns them to a negative bug id.
However, sheriffs should have the option of recording a message when they ignore alerts.
Ignoring alerts should instead assign them to a new bug marked WontFix, that can contain a message from the sheriff. This will also record who ignored the alerts to facilitate further discussion.

Charts currently display different icons for ignored alerts versus assigned alerts because it's easy to tell whether a bug id is negative. Maybe at some point the dashboard (both alerts-table and chart icons) can query for a bug's current status to indicate whether the bug is currently WontFix, regardless of how it came to be so. In the short term, I think it's probably fine to display the same icon for triaged alerts and ignored alerts, and allow users to click through to crbug to see the current status.