Update/Add License information to source code

All files (where possible) in https://github.com/origam/origam-html5 have to have the following license info in the beginning of the file:

/*
Copyright 2005 - 2021 Advantage Solutions, s. r. o.

This file is part of ORIGAM (http://www.origam.org).

ORIGAM 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.

ORIGAM 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 ORIGAM. If not, see <http://www.gnu.org/licenses/>.
*/

Implemented in

@jsusen Please do the same for GitHub - origam/origam-html-chat

Please also bump the year (2020->2021) in GitHub - origam/origam-source. Both license headers and assemblyInfo.cs