(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.

Change-Id: I72d671712c000cce2831770895e06626c0fa6f51
Merge "(bug 40157) store entity status in page_props"