Use relative path link for system notes in the GitLab UI

In the GitLab UI, use a relative _path helper in system notes like new_issue_branch but these also may be returned in the API which probably should be absolute URLs

I need to check if the message is stored(tricky) or always generated. We could pass a only_path: true-like option(like we do for `avatar_url).

This is especially important for both Geo and customers using a subpath in their external_url.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15738#note_50077405

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Discovered while working on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15738