Check the content model of pages in EntityPerPageBuilder

Just to make sure the script isn't going to choke on wikitext
pages in the item NS.

This commit introduces WikibaseRepo::getContentMappings() which
should be made us of.

Change-Id: I9e0a5ca212d3ce210d1ca42a4829abf86ae18cde
Merge "Check the content model of pages in EntityPerPageBuilder"