Refactor invoice PDF generation, download and delivery

- Remove `Que::Mailer` (#895)
- Extract controllers
- Extract translations
- Convert HAML to ERB
- Add mailer preview
- Improve UI
- Remove unused routes
- Add tests

app/controllers/admin/invoices/delivery_controller.rb

@@ -0,0 +1,13 @@
+module Admin
+  module Invoices
+    class DeliveryController < BaseController
+      include Deliverable
+
+      private
+
+      def redirect_url
+        admin_invoice_path(@invoice)
+      end
+    end
+  end
+end