WIP: Group MR numbers on issue show page by project and namespace

What does this MR do?

Groups MRs on the issue show page into those for the current project, those for projects in the namespace, and projects outside of the namespace. This makes the ordering consistent with the work done on #7110.

Are there points in the code the reviewer needs to double check?

None that I can think of. Maybe if anything, a visual separator between different projects?

Why was this MR needed?

Consistency with other ordering.

Screenshots (if relevant)

Old: New:

Does this MR meet the acceptance criteria?

• Changelog entry added
• N/A Documentation created/updated
• N/A API support added
• Tests
  • N/A Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26466 (moved)