CI: use check_mr_settings.py from ci-common.git

While working on #521 (closed), I've realized that we don't use the shared version of check_mr_settings.py in pmaports.git yet. Let's fix this.

Submitting this MR first without "Allow commits from members who can merge to the target branch." to show that it works: https://gitlab.com/ollieparanoid/pmaports/-/jobs/598046214 (failed, as expected)

After merge, cherry-pick to v20.05 branch.