Add notification when emergency shutoff is used

[jcristau]

Emergency shutoff can be made on the firefox release channel with no dual signoff (on purpose, since it's an emergency tool), and currently doesn't notify anyone, so updates can potentially stay disabled without detection for some time. We should make it notify someone (via matrix? slack? email?).

[hwine]

Great idea! Please check with the SIRT team to see if this is the sort of message they want to alert on -- that may influence which channel is best.