In header comments, update GNU links with https

In each Moodle file, there are links in the header comments to GNU site and the GPL licence.

Now that most sites are served by https, i think we should update links to reflect that.

So, i propose that we update all links :

http://www.gnu.org/licenses/ to https://www.gnu.org/licenses/

http://www.gnu.org/copyleft/gpl.html to https://www.gnu.org/licenses/gpl-3.0.html

We could also replace "This file is part of Moodle - http://moodle.org/" by "This file is part of Moodle - https://moodle.org/"