Add licensing for extensions to Special:Version

Allow extensions to register a software license and present
this on the Special:Version page. A new $wgExtensionCredits
parameter has been introduced for this purpose: 'license-name'.

This will also automatically pick up the presense of additional
licensing and/or credits files. If ((AUTHORS)|(CREDITS))(\.txt)?
exists in the extension base directory a credits link will be
created. If ((LICENSE)|(COPYING))(\.txt)? exists a license link
will be created.

The API has also been updated to produce VCS information and present
links to the license/credits files.

Bug: 48418
Change-Id: I388f3b630462f1909f30751c987f7af585e98881
Merge "Add licensing for extensions to Special:Version"