Add file-link "parser" to mw.Title from commons

Rillke has been amazing and provided us with a first pass at a file URL
parser for MediaWiki:

https://commons.wikimedia.org/wiki/MediaWiki:Gadget-libUtil.js

Hoping to use this for Extension:MultimediaViewer and others in the near
future - potentially replacing patchset Ib69d1ffdfd3b58c2e8e3314940a2cad311b85533

Also tests.

Change-Id: Ie31e6df376bda087e92ec889d54df3374ba96b5e
Merge "Add file-link "parser" to mw.Title from commons"