R1:8d24e3a21a73

When a dashboard has inconsistent panel policies: keep doing nothing

Summary:
Depends on D20376. Ref T8033. It's possible to put a bunch of secret panels on a public dashboard, and not obvious that the dashboard won't be very useful.

This was more of an issue long ago (when the dashboard broke or all the panels completely vanished or something?). Nowadays, the panels render "You don't have permission to view this" so it's likely easy to explain/fix. Still, we can warn about this.

But, for now, don't…