(bug 44108) Honor $wgSend404Code for missing entities.

A request for an entity page that does not exist will now return a 404
if $wgSend404Code is true.

This change also adds the deletion log excerpt that is shown when visiting
a deleted wiki page.

Change-Id: I14be91ead06e746da7ef7ac2d99df21252f26469
Merge "(bug 44108) Honor $wgSend404Code for missing entities."