GitLab project path is not resolved correctly when self-hosted GitLab is not in the domain root

If self-hosted GitLab is not in the domain root, the project path is not resolved correctly and leads to 404 responses.

For example if instanceUrl is set to http://example.com/gitlab, the /gitlab part will be included when requesting the project from the GitLab API (http://example.com/api/v4/projects/gitlab%2Fgroup%2Fproject).