Fix budget investments chart in admin stats

The JavaScript required to display the chart wasn't loaded on the admin
stats page.

We're not adding a test because we're going to move the budgets graph to
a different page on the pull request containing this commit.

Note we're changing the "Go back" link, since using a turbolinks refresh
broke this link when using the Chromium browser. Besides, there was an
inconsistency where some of the "Go back" links in admin stats pointed
to the admin stats page but other links pointed to `:back`.

spec/system/admin/stats_spec.rb

@@ -183,6 +183,8 @@ describe "Stats", :admin do
       visit admin_stats_path
       click_link "Proposal notifications"
 
+      expect(page).to have_link "Go back", href: admin_stats_path
+
       within("#proposal_notifications_count") do
         expect(page).to have_content "3"
       end
@@ -232,6 +234,8 @@ describe "Stats", :admin do
       visit admin_stats_path
       click_link "Direct messages"
 
+      expect(page).to have_link "Go back", href: admin_stats_path
+
       within("#direct_messages_count") do
         expect(page).to have_content "3"
       end
@@ -253,6 +257,8 @@ describe "Stats", :admin do
         click_link "Polls"
       end
 
+      expect(page).to have_link "Go back", href: admin_stats_path
+
       within("#web_participants") do
         expect(page).to have_content "3"
       end