Deleting source project with existing fork link should close all related merge requests
Summary
Now when source project with existing fork link is deleted all merge requests stay open. Fore those merge request is rendered invalid template (https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/projects/merge_requests/invalid.html.haml#L1). In this view we have two buttons "Close" and "Edit" MR. Only "Close" works, "Edit" causes an error.
Expected behavior
When source project with existing fork link is deleted it should close all related merge requests. It would be similar behaviour to deleting fork link between projects, and we could get rid of this invalid template. Thanks to !5496 (merged) we can still edit merge request without source project.