Improvement: separate permissions to read assets vs to list them

[sfirke]

My public role needs the can read on dashboard permission to be able to see dashboards, but this permission also gives them access to https://superset.example.com/dashboard/list/ that I would prefer to disable.

It's a small thing, but it would be nice to separate those permissions into can read on dashboard for accessing dashboards and a new one, can list on dashboard. Same for can read on chart and can read on dataset.