Merge request discussions can't be resolved when running GitLab at a relative URL

Summary

The merge request "Resolve discussion" button and checkmarks for comments have no effect when running GitLab at a relative URL. The URL used for the POST request doesn't contain the relative URL part.

If I edit the URL and resend the request, the discussion is resolved correctly.

GitLab Version: 8.11.3 (CE)

Steps to reproduce

1. Run GitLab at a relative URL (ex. https://server/path/to/gitlab)
2. Attempt to resolve a discussion or comment on a merge request

Expected behavior

POST https://server/path/to/gitlab/group/project/merge_requests/1/discussions/hash/resolve

Actual behaviour

POST https://server/group/project/merge_requests/1/discussions/hash/resolve