If user can see table but NOT database/instance nav links should not display

Spotted this bug while building this plugin:
- https://github.com/simonw/datasette-public

This is a public table, but the two links in the nav go to forbidden pages:

<img width="1007" alt="image" src="https://user-images.githubusercontent.com/9599/193964983-c574efcd-fd7d-4507-bb71-58fe5252a831.png">

Those nav links shouldn't be shown at all.