Make “closing issue” UI more discoverable in merge request

Currently when you want to indicate a merge request closes an issue (a GitLab issue or an external integration like Jira), you write closes #123. This isn’t very discoverable because there’s no way to know that this feature exists. But once you do do it, it’s immediately visible in the mr widget itself.

How can we make this more obvious?

I think in addition to parsing the mr description, you could allow a user to paste in an issue id (or external URL) in the mr widget itself.