- 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.