dashboard next: perf and UI tweaks

* cache CORE reports
* adds backups/uploads section
* few css tweaks

app/controllers/admin/dashboard_next_controller.rb

@@ -1,2 +1,9 @@
+require 'disk_space'
 class Admin::DashboardNextController < Admin::AdminController
+  def index
+    dashboard_data = AdminDashboardNextData.fetch_cached_stats || Jobs::DashboardNextStats.new.execute({})
+    dashboard_data.merge!(version_check: DiscourseUpdates.check_version.as_json) if SiteSetting.version_checks?
+    dashboard_data[:disk_space] = DiskSpace.cached_stats
+    render json: dashboard_data
+  end
 end