XD-386(kinda): Add copyright header to files that miss it

Still can't apply https://github.com/hierynomus/license-gradle-plugin as it suffers from https://github.com/hierynomus/license-gradle-plugin/issues/39 (and we don't want headers on **all** our files, including txt, xml, properties, etc)

So configured exclusions and ran it once by hand for now.