R1:70d67a3908b8

Fix the most significant "phantom notification" badness

Summary:
Ref T13124. Ref T13131. Fixes T8953. See PHI512.

When you receieve a notification about an object and then someone hides that object from you (or deletes it), you get a phantom notification which is very difficult to clear.

For now, test that notifications are visible when you open the menu and clear any that are not.

This could be a little more elegant than it is, but the current behavior is very clearly broken. This unbreaks it, at…