Updated and enhanced the extension

This extension was only available in-wiki at
https://www.mediawiki.org/wiki/Extension:MobileDetect
I'm uploading it to Gerrit for the first time, after updating
its code structure, while keeping backwards compatibility,
and enhancing it with the tags <nomobile> and <mobileonly>

Change-Id: I85cca8345fcbd27499508ad687028771f06766ca