(bug 40157) store entity status in page_props
This adds the following properties to page_props for entity pages:
wb-status (may indicate empty, stub or other), wb-claims (the number of
claims) and, for items, wb-sitelinks (the number of sitelinks).
DEPLOYMENT: we'd need a script to populate the page_props for existing
items to make this useful. We can deploy without that, though.