jshint: Allow global mw to be set in mediawiki.js

In master this has been cleaned up, but in REL1_20 we still used
the 'mw' global directly in a bunch of placed so we need to tell
jshint that it is okay for it to be set in mediawiki.js

It is currently complaining about redefinition because of this.

Change-Id: Ifb55efae5f4ff10ffe1873bdf02d43aa2d61857a