patch against r61453
[[Special:Version]]'s list of extensions includes links to the mediawiki.org page documenting each extension; however, this information is missing from the API's equivalent siprop=extensions output. Often the URL is obvious (e.g. ConfirmEdit is at Extension:ConfirmEdit) but sometimes it is not, as with the many extensions that are part of the usability initiative.
The attached patch adds a "url" attribute to the output for any extensions for which a URL is defined.