Use relative path link for Issue discussion note avatars
Use relative path link for Issue discussion note avatars. This stems from <namespace>/<project>/issues/<iid>/discussions.json
returning absolute URL author avatars.
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
Discovered while working on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15738