REUSE: Giant boilerplate cleanup

- point to main Calamares site in the 'part of' headers instead
  of to github (this is the "this file is part of Calamares"
  opening line for most files).
- remove boilerplate from all source files, CMake modules and completions,
  this is the 3-paragraph summary of the GPL-3.0-or-later, which has
  a meaning entirely covered by the SPDX tag.

lang/txload.cpp

@@ -1,21 +1,10 @@
-/* === This file is part of Calamares - <https://github.com/calamares> ===
- *
- *   Calamares 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.
- *
- *   Calamares 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 Calamares. If not, see <http://www.gnu.org/licenses/>.
+/* === This file is part of Calamares - <https://calamares.io> ===
  *
  *   SPDX-FileCopyrightText: 2018 Adriaan de Groot <groot@kde.org>
  *   SPDX-License-Identifier: GPL-3.0-or-later
- *   License-Filename: LICENSE
+ *
+ *   Calamares is Free Software: see the License-Identifier above.
+ *
  */
 
 /*