gitlab: convert old REVIEWERS into GitLab's CODEOWNERS
See https://docs.gitlab.com/ce/user/project/code_owners.html
This MR contains !2177 (merged) but I'm happy to land it as a simple new file it we decide to keep the old reviewers script in place.
See https://docs.gitlab.com/ce/user/project/code_owners.html
This MR contains !2177 (merged) but I'm happy to land it as a simple new file it we decide to keep the old reviewers script in place.