CODEOWNERS: add maintainers from APKBUILDs + CI test

See individual commits for details.

Sometimes we need to figure out the gitlab nicknames of maintainers and
co-maintainers as listed in the APKBUILDs, e.g. when a specific issue
appears that only happens with a given device. Or if we want to ping
all device maintainers in the testing phase for the new release.

So far it has been a very tiresome, manual process to figure out gitlab
nicknames from the maintainers in the APKBUILDs (involving git log
--follow and opening merge requests in browsers...). After some
discussion we figured it would be good to store it in the CODEOWNERS
file.

This also has the nice side-effect that maintainers get notifications if
the APKBUILDs they maintain get changed.