Markdown rendering: use absolute repository links

What does this MR do and why?

This MR solves #382171 (closed).

🛠 with ❤ at Siemens

/cc @bufferoverflow

How to set up and validate locally

See #382171 (closed)

You can send a notification email for a comment of a merge request (note):

note = Project.find(7).merge_requests.last.notes.first # Note.find(id)
Notify.send(:note_merge_request_email, recipient.user.id, note.id, "mentioned").deliver_now

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.