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.

app/controllers/admin/stats_controller.rb

@@ -75,14 +75,6 @@ class Admin::StatsController < Admin::BaseController
     @budget = Budget.find(params[:budget_id])
 
     authorize! :read_admin_stats, @budget, message: t("admin.stats.budgets.no_data_before_balloting_phase")
-
-    @user_count = @budget.ballots.select { |ballot| ballot.lines.any? }.count
-
-    @vote_count = @budget.lines.count
-
-    @vote_count_by_heading = @budget.lines.group(:heading_id).count.map { |k, v| [Budget::Heading.find(k).name, v] }.sort
-
-    @user_count_by_district = User.where.not(balloted_heading_id: nil).group(:balloted_heading_id).count.map { |k, v| [Budget::Heading.find(k).name, v] }.sort
   end
 
   def polls