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

app/views/layouts/nav/_project.html.haml

app/views/projects/labels/edit.html.haml

app/views/projects/labels/index.html.haml

app/views/projects/labels/new.html.haml

app/views/projects/merge_requests/_head.html.haml

app/views/projects/merge_requests/index.html.haml

app/views/projects/milestones/edit.html.haml

app/views/projects/milestones/index.html.haml

app/views/projects/milestones/new.html.haml

app/views/projects/milestones/show.html.haml

app/views/shared/_mr_head.html.haml

changelogs/unreleased/30779-show-mr-subnav-issue-tracker.yml