Allow cross-project merge requests

Description

For some projects, changes of a merge request end up in multiple projects. It would be nice to group merge requests such that they have discussion about code changes visible within all the projects.

Proposal

I have no idea of how this feature would look like in the UI. Maybe it is as simple as having related merge requests and a "create merge request from changes of other merge request"-button.

Use cases

One use case is GitLab CE / EE which mostly have the same changes being merged in. With this feature I imagine changes being merged to both projects from the same place, so we would not need upstream merges anymore.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.