This adds SPDX license headers to all source code files, following the
REUSE[1] specification. This does not include any asset files (such as
generated JSON files, or textures). While REUSE does support doing so
with ".license" files, for now we define these licences using the
.reuse/dep5 file.
[1]: https://reuse.software/
In classic squid tradition: 20% code, and 80% test logic.
Closes#962. Alas, whoever reported this has deleted their account, so
they can't even be happy about it :(.
See comments in c8c128d335385d5afd6857dc25cc090672cc8563 for further
details. This requires /relatively/ few changes - mostly cases we were
missing @Nullable annotations.