Moves it from Settings to Common::TimeZone, since this algorithm doesn't
depend on the setting. It also lets us use it in other libraries.
common: Various fixes
time_zone: Don't double up the std::abs
Too many absolute values were causing mirrored time zones to resolve
as the same.
This formats all copyright comments according to SPDX formatting guidelines.
Additionally, this resolves the remaining GPLv2 only licensed files by relicensing them to GPLv2.0-or-later.
Now that clang-format makes [[nodiscard]] attributes format sensibly, we
can apply them to several functions within the common library to allow
the compiler to complain about any misuses of the functions.
