Add PagesWithProp special page and API module

It would be useful to be able to list pages using a particular page
property, particularly in light of the new Disambiguator extension.

This adds a special page Special:PagesWithProp and an API query module
list=pageswithprop to do just this. It also adds an API query module
list=pagepropnames to list the prop names currently in use on the wiki.

Change-Id: Ib0d4e17f22b8d0cb9894eac6095962315480e809
Merge "Add PagesWithProp special page and API module"