Extract component for balloting budget admin stats

We're also moving the tests, but we're keeping one system test in order
to test the controller and the navigation to get to this page.

Note we're slightly changing the order of the methods in the component;
the order of the instance variables was `user_`, `vote_`, `vote_`,
`user_`, which was hard to follow.

spec/system/admin/stats_spec.rb

@@ -195,19 +195,6 @@ describe "Stats", :admin do
         end
 
         expect(page).to have_content "VOTES\n3"
-      end
-
-      scenario "Number of users that have voted a investment project" do
-        create(:user, ballot_lines: [investment])
-        create(:user, ballot_lines: [investment])
-        create(:user)
-
-        visit admin_stats_path
-        click_link "Participatory Budgets"
-        within("#budget_#{budget.id}") do
-          click_link "Final voting"
-        end
-
         expect(page).to have_content "PARTICIPANTS\n2"
       end
     end