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

config/locales/en/admin.yml

@@ -38,6 +38,8 @@ en:
           sdg: SDG
       create:
         notice: "Banner created successfully"
+      destroy:
+        notice: "Banner deleted successfully"
       edit:
         editing: Edit banner
         form:

config/locales/es/admin.yml

@@ -38,6 +38,8 @@ es:
           sdg: ODS
       create:
         notice: "Banner creado correctamente"
+      destroy:
+        notice: "Banner eliminado correctamente"
       edit:
         editing: Editar el banner
         form:

spec/system/admin/banners_spec.rb

@@ -177,6 +177,8 @@ describe "Admin banners magement", :admin do
       click_button "Delete"
     end
 
+    expect(page).to have_content "Banner deleted successfully"
+
     visit admin_root_path
     expect(page).not_to have_content "Ugly banner"
   end