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

Kushal Pandya requested to merge 30779-show-mr-subnav-issue-tracker into master

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?

What are the relevant issue numbers?

Closes #30779 (closed)

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