Migrate license from GPL to Apache 2.0

I've been unable to get a response from Matt in the past few days. I assume that the licence header on admin pages should indeed be changed too. They are, after all, part of the product that will be released under the Apache license.

Done for /src/web/ There are a couple of plugins left. Should we update their version number, if we modify the license information?

I remember that you already updated a couple of the plugins with a new version for the SLF4J usage. I would vote to update the version of the remains if there are any. And by the way maybe we should also reflect the the new version 3.7.0 instead of 3.6.5 (minServerVersion in the plugin.xml files).

Good point. I just made these changes.

I think we're there.