(bug 47070) check content model namespace on import.

When importing, we need to check that the kind of content we are about to
import is actually allowed in the specified location on the local wiki.

This change does two things:

* Introduce the ContentModelCanBeUsedOn hook which provides control over
which kind of content can be used where.

* Introduce a check against ContentHandler::canBeUsedOn in the importer,
along with an appropriate error message.

Change-Id: Ia2ff0b0474f4727c9ebbba3c0a2a111495055f61