Hide all existing notifications when do-not-disturb is turned on

We maybe should hide existing notifications when Do Not Disturb is turned on, with the exception of critical notifications. This would allow users to implement a sort of "boss key" that can hide their notifications when someone walks up to their desk (and, with DnD, ensure no new notifications pop up). Since this would be a behavior change, hopefully it's not something that would make existing users upset. I'd really prefer to not have a user preference for this, though maybe that wouldn't be the end of the world.