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.

src/modules/mount/main.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
-# === This file is part of Calamares - <https://github.com/calamares> ===
+# === This file is part of Calamares - <https://calamares.io> ===
 #
 #   SPDX-FileCopyrightText: 2014 Aurélien Gâteau <agateau@kde.org>
 #   SPDX-FileCopyrightText: 2017 Alf Gaida <agaida@siduction.org>
@@ -9,18 +9,8 @@
 #   SPDX-FileCopyrightText: 2019 Kevin Kofler <kevin.kofler@chello.at>
 #   SPDX-License-Identifier: GPL-3.0-or-later
 #
-#   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 Free Software: see the License-Identifier above.
 #
-#   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/>.
 
 import tempfile
 import subprocess