[FEAT] Site: Return the default searched namespaces

This returns a set of the namespaces which are searched when the
namespaces are not given explicitly. If not logged in they'll return
the default namespaces set for the server and otherwise the
namespaces selected in the user preferences.

Change-Id: Ieed1f9890d4e7dc631d6e2d33c776592eda2c17f