Visibility restrictions for the API only

[muhme]

Currently, non-public categories are visible in the list view without login, but access is denied in the detail view. This is not consistent. And since we now also have comments that are always visible the same way, access should now become consistent.

Non-public quotes, categories and authors should be findable in the lists, detail views and with the search. Only the playout on the websites shall be prevented.

The tests and the documentation are to be adapted.