Move ExpandTemplates special into core

Very little modification to extension, add some docs, and GNU license as
per the other core extensions. Removed $isNewParser, as it won't be needed
with new versions of MW, and changed the name of POST variables used.
I have not moved across any of the messages except
english, according to the bug report, translatewiki will sort this out.
The expandtemplates message section has also been added to messages.inc.

htmlspecialchars should not be performed on anything, as it will cause it
to render like &lt;root&gt;

Added this change into the 1.23 release notes.

Bug: 28264
Change-Id: I7ef63488dc3ad3885bcf99ff52852e1c6981942b
Merge "Move ExpandTemplates special into core"