Replace various "active tab" checks with nav_link

Also remove now-unused tab_class helper

app/views/tree/_head.html.haml

@@ -1,9 +1,8 @@
 %ul.nav.nav-tabs
   %li
     = render partial: 'shared/ref_switcher', locals: {destination: 'tree', path: @path}
-  %li{class: "#{'active' if (controller.controller_name == "tree") }"}
-    = link_to project_tree_path(@project, @ref) do
-      Source
+  = nav_link(controller: :tree) do
+    = link_to 'Source', project_tree_path(@project, @ref)
   %li.right
     .input-prepend.project_clone_holder
       %button{class: "btn small active", :"data-clone" => @project.ssh_url_to_repo} SSH