Change license to GPLv3 or newer (Task #1105)

ChiliProject (and Redmine) is currently licensed under the GPLv2 or newer (see Redmine's license and ChiliProject's license)

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

We now started to look into using additional libraries, some of which are licensed under Apache 2.0, e.g. Twitter Bootstrap and Select2. Unfortunately, the GPLv2 is not compatible with the Apache 2.0 license, while the GPLv3 is compatible according to http://www.gnu.org/licenses/license-list.html#apache2 This means, if we want to use these Libraries in the core and want to ship them, we have to drop the GPLv2 and move to the GPLv3 or later.

For our own code, this is easily possible, as we can freely chose any later version from the GPLv2. I assume that all contributors are licensing their contributions under the terms of the main ChiliProject license, i.e., GPLv2 or later. If that wasn't the case, we (and Redmine) would not be able to ship anything at all. However, we have to check all libraries shipped with the core if they are compatible with the GPLv3.

The steps we have to perform thus are:

1. Check all libraries in the core, whether referenced in the Gemfile or vendored in lib or vendor/plugins
   • Remove or update any libraries incompatible with the GPLv3
2. Inform plugin authors about the license change ahead of time. They might have to update their licenses too.
   • E.g. the popular redmine_backlogs plugins appears to be licensed under GPLv2 only.