Added square caret to Back to Settings button

This was done to show difference between Open/Close toggle, which
was using the same arrow as Back to Settings button previously.

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

@@ -2,7 +2,7 @@
   - if @project_settings_nav
     = nav_link do
       = link_to namespace_project_path(@project.namespace, @project), title: 'Back to project', class: "" do
-        %i.fa.fa-angle-left
+        %i.fa.fa-caret-square-o-left
         %span
           Back to project