Resolve "Move sub_nav content above the content-wrapper class"

This MR moves the sub_nav content above the sidebar so that the sub_nav can be full-width without resorting to hacks. This is necessary prerequisite for !10096 (merged)

Closes #29880 (closed)

spec/features/projects/members/user_requests_access_spec.rb

@@ -61,7 +61,7 @@ def open_project_settings_menu
       click_link('Settings')
     end
 
-    page.within('.page-with-layout-nav .sub-nav') do
+    page.within('.sub-nav') do
       click_link('Members')
     end
   end