[11.10] Use a path for the related merge requests endpoint
Hosts using a non-standard configuration may have a different hostname/port/scheme than what may be configured on the GitLab server. While expose_url should generate a proper URL, there are cases where it may not work. Since we don't need the full URL, we can use the relative path.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61280
This backports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28171 to 11.10.
Edited by Stan Hu