Resolve "Fix image not rendering in markdown after drag and drop in drawer"

What does this MR do and why?

This merge request calculated the upload path for the markdown editor in the drawer depending on the drawer work item namespace rather than using the default window.uploads_path for it. The latter caused an error when we opened a drawer with an issue (project namespace) in the group context (epic, group issues board or list)

References

• #520607 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Enable new issues experience.
2. Open the issue in the drawer in group context (group board, group list, epic's child items)
3. Try pasting an image to the comment / work item description
4. Open any other drawer (epic, project issue)
5. Try pasting an image again to comment and description

Related to #520607 (closed)