Require a ; after RETRANSLATE macros. They are statement-like;
this makes it easier for some of them to be recognized by
clang-format and resolves some existing weird formatting.
- 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.
