[All] Add licensing info

[jweile]

https://www.gnu.org/licenses/agpl.txt

https://www.gnu.org/licenses/gpl-howto.html :

• Add a COPYING file with a copy of the GNU GPL or GNU AGPL.
• Put a license notice in each file:

Copyright 1998, 1999 Joe User
This file is part of Foobar.

Foobar 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 3 of the License, or
(at your option) any later version.

Foobar is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with Foobar. If not, see https://www.gnu.org/licenses/.