Fix cross-project references copy to include the project reference

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25890

So the project reference is included while copy the link to issues or MRs.

• Changelog entry added
• Tests
  • 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?