Fix markdown import attachments in GitHub Import

What does this MR do and why?

Related to: #385204 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Create a GitHub project. Create notes (comments)/release description with attachments. After import make sure that notes/release posts have attachment links that belongs to gitlab server.

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.