license: Add COPYING file for GNU GPL v2

The only file with a license mention so far was ApiTemplateData.php.
It was documented on mediawiki.org[1] as "GPL v2".

[1] https://www.mediawiki.org/wiki/Extension:TemplateData

Change-Id: I85cdd5d511f8578888127888d11c2642aad49842
Merge "license: Add COPYING file for GNU GPL v2"