Updated mediawiki/extensions
Project: mediawiki/extensions/MobileFrontend  38d4ea36690dc7857be4ac761deac48b02e4290e

Fix grunt.initConfig so it uses MEDIAWIKI_URL

This is the env variable that we already use elsewhere.

Also adding to documentation.

Change-Id: Icfef88c40473674741e2f31151c9181f405a42da