Add notice after destroying a banner

We were missing a notice in this case. Not only this caused
inconsistencies in the user experience, but it also made it hard to add
an expectation in the test checking the request had finished before
making a new one. Simultaneous requests sometimes cause failures in our
test suite.

app/controllers/admin/banners_controller.rb

@@ -32,7 +32,7 @@ class Admin::BannersController < Admin::BaseController
 
   def destroy
     @banner.destroy!
-    redirect_to admin_banners_path
+    redirect_to admin_banners_path, notice: t("admin.banners.destroy.notice")
   end
 
   private