Use confirmCloseWindow from mediawiki-core

Depends on I5d35b6fddd73cf56371eeda299ee0779cb40397a
See the card at http://fab.wmflabs.org/T517 

Change-Id: I9411a6d338eb40de622ccab2b07b89c6f60dbebc
Merge "Use confirmCloseWindow from mediawiki-core"