(bug 26280) all files should have GPL header

(bug 26280) all files should have GPL header

MediaWiki is licensed under GPL so we should really add the header to
our PHP code unless reusing code licensed differently.

Tested using:
  ack-grep --php -L 'This program is free software' \
    includes/ maintenance/ mw-config/ resources/ skins/

Output the following files:
 includes/ExternalEdit.php (public domain)

Various other licenses:
 includes/json/Services_JSON.php
 includes/libs/CSSMin.php
 includes/libs/IEContentAnalyzer.php
 includes/libs/JavaScriptMinifier.php
 includes/libs/jsminplus.php
 includes/normal/Utf8Case.php
 includes/objectcache/MemcachedClient.php

File autogenerated from a GPL file and Unicode data:
 includes/ZhConversion.php

A few lines coming from stack overflow:
 maintenance/mwdoc-filter.php

Message files do not have any license :-/

Change-Id: I214a988edfa9e2dfcc762612a0f7d47eee8bcb74