Render issues link on issues subnav unless you visit merge request controller

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

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

 %ul.nav-links.sub-nav
   %div{ class: (container_class) }
-    - if project_nav_tab?(:issues) && current_controller?(:issues)
+    - if project_nav_tab?(:issues) && !current_controller?(:merge_requests)
       = nav_link(controller: :issues) do
         = link_to url_for_project_issues(@project, only_path: true), title: 'Issues' do
           %span