Replace absolute URLs on related branches/MRs with relative url to avoid hostname

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3626

• Replace absolute URLs on related branches/MRs with relative url to avoid hostname

Are there points in the code the reviewer needs to double check?

I tested this with a relative URL but I don't think that is the actual issue because it works before this fix.

I think the problem is the same as https://gitlab.com/gitlab-org/gitlab-ce/issues/39364 and the hostname not being honored with the _url helper. So we switch the to relative _path helpers which do not include the hostname and handle a relative URL just fine.

Why was this MR needed?

When using a different hostname, the URL would point to something that doesn't exist (404). In the local development case, this may work because localhost still responds.

I think this was broken since 8.7 when we started using the _url helpers for related MRs/branches, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3678/diffs#ad9731dd71c7dbcf115e11bf539028679eba6fe9_70_70 - Is this reasonable or is the hostname changed often and someone would have run into this sooner?

https://gitlab.com/gitlab-org/gitlab-ce/issues/40555#note_50056843

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
  • Do we have tests for a relative URL setup somewhere?
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #40555 (closed)