Adding missing contributors to COPYRIGHT.txt

It appears a few ones fell through the cracks. Running

git log --pretty=format:"%an%x09" . | sort | uniq

Shows a few names that don't show up in the COPYRIGHT.txt files. Note the output from the command cannot easily taken as is, because some commits are not authored with the actual name but just an e-mail address (and we usually looked up the full name on GitHub manually then); also it contains accounts like "Jenkins User".