Chain GitLab events

As one of the release manager for 9.3, I often found myself doing stuff like:

Steps

1. If https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2195 MWPS and merges
2. Then Merge https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2186
3. Then Merge

Kind of like IFTT or Zapier for gitlab events. I think this would be a nice EE feature and can help streamline a lot of the manual processes done by release managers