UX: Improve rough edges of AI usage page (#1014)

* UX: Improve rough edges of AI usage page

* Ensure all text uses I18n
* Change from <button> usage to <DButton>
* Use <AdminConfigAreaCard> in place of custom card styles
* Format numbers nicely using our number format helper,
  show full values on hover using title attr
* Ensure 0 is always shown for counters, instead of being blank

* FEATURE: Load usage data after page load

Use ConditionalLoadingSpinner to hide load of usage
data, this prevents us hanging on page load with a white
screen.

* UX: Split users table, and add empty placeholders and page subheader

* DEV: Test fix

app/controllers/discourse_ai/admin/ai_usage_controller.rb

@@ -6,6 +6,9 @@ class AiUsageController < ::Admin::AdminController
       requires_plugin "discourse-ai"
 
       def show
+      end
+
+      def report
         render json: AiUsageSerializer.new(create_report, root: false)
       end