Show sub-nav under Merge Requests when issue tracker is non-default

What does this MR do?

When project has 3rd-party Issue tracker integration, clicking on Issues tab will directly take user to issue tracker page. Starting %9.0 we changed navigation order for Projects (see #26348 (closed)), where we removed sub-nav items from Merge Requests tab and showed them under Issues tab. In this case, user will never see sub-nav since page is redirected to 3rd-party issue tracker.

This MR restores sub-nav under Merge Requests when project has non-default Issue tracker (eg; JIRA, Bugzilla) integrated with the project. See #30779 (closed).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support 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?

Closes #30779 (closed)