Add .jshintrc for Wikibase and make it pass

Basically follows the current MediaWiki conventions, except where the codebase diverges due to
recent changes in the conventions.

Authors: Marius Hoch, Daniel Werner

Change-Id: Ie39d21c80afd08af9e5fa40765bc454100c51ba6
Merge "Add .jshintrc for Wikibase and make it pass"