Trigger GitLab CI runs for external PRs

[austince]

GitLab CI works by mirroring the GitHub repo but this does not allow external PRs to trigger CI pipelines, as the PR itself is not mirrored.

Coming from this suggestion, we should add a GH Action to trigger GitLab CI jobs for external PRs. Need to see if this will work for forks making PRs.

See:
https://github.com/marketplace/actions/trigger-gitlab-ci

[austince]

Could also just switch to GitHub Actions CI/ CD, to keep it all here.