Add Gitlab CI to check Copyright

This MR adds the Gitlab CI check, that was missing on 2017-patches branch. Adding the CI check solves the issue that MRs on this branch could not be merged, because they were waiting for completion of a non-existent CI check!

Note that, until the Copyright is added systematically to all files on this branch, it will have to be added ad hoc to any files that are modified by subsequent MRs.