Add an indicator in the editor's list view when a block has visibility restrictions

[jeremyfelt]

Given the structure in this screenshot, we had an instance where date/time visibility was applied to a top level group. The start time passed and content did not show as expected. After digging, we found that date/time visibility from a year earlier was also set on a group further down the chain. That happened either because of a copy/paste from an earlier page with similar content or a reusable block.

This in itself is not really preventable, but the issue would be easier to diagnose if there was some way of highlighting which blocks had visibility restrictions without going through each.

That all said... the editor UI is confusing enough with all its various bits and bobs, so this might not be the right place to try adding another icon. But! I'm open to help try if this sounds interesting. 😄