Prevent MRs with specific labels to be merged

Use case

There may be times where you create MRs to preview things, but don't want to merge them.

Specifically, in my use case, I have https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/67 where it only serves as a Review app and I labeled it as Preview only. Once it serves its purpose I'm gonna close it.

Not sure if it's worth the effort though, since we have WIP MRs that prevent you to merge them.

cc @mydigitalself