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, since a lot of this works better now and it's not really clear that this is particularly valuable. We can revisit this after all the connected changes have more of a chance to settle.

Test Plan:
(Earlier behavior, not how things look in the final version.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T8033

Differential Revision: https://secure.phabricator.com/D20377