Fix "Go back" link in stat graphs

The link appeared twice when visiting stats for a budget's supporting
phase.

app/views/admin/stats/_graph.html.erb

@@ -1,5 +1,3 @@
-<%= back_link_to %>
-
 <div id="graph" class="small-12 column">
   <h2><%= t "admin.stats.graph.#{name || event}" %> (<%= count %>)</h2>
   <%= chart_tag id: name, event: event %>

app/views/admin/stats/graph.html.erb

@@ -2,4 +2,6 @@
   <%= javascript_include_tag "stat_graphs", "data-turbolinks-track" => "reload" %>
 <% end %>
 
+<%= back_link_to admin_stats_path %>
+
 <%= render "graph", name: @name, event: @event, count: @count %>

spec/system/admin/stats_spec.rb

@@ -109,6 +109,7 @@ describe "Stats", :admin do
         end
 
         expect(page).to have_content "Votes 3"
+        expect(page).to have_link "Go back", count: 1
       end
 
       scenario "Number of users that have supported an investment project" do