(bug 47070) No import of wikitext to entity NS

This uses the new ContentModelCanBeUsedOn hook to control which
kind of content can be placed in entity namespaces.

This should prevent e.g. wikitext to be imported into the main
namespace.

IMPORTANT: does nothing without Ia2ff0b0474f4727 in core.

Change-Id: Id1fe6abf9ea843cf23bf79857673227fc46c178d
Merge "(bug 47070) No import of wikitext to entity NS"