Add functionality to delete retired apps

Retired apps are being flagged in the Out of sync release
alert and there is no way to remove them in the Release app.

This commit allows an app to have the option of being deleted on
the Edit page. There is a popup confirmation dialog when the delete
button is pressed to ensure it is not removed by mistake.

app/assets/javascripts/application.js

@@ -1,3 +1,5 @@
 //= require_tree .
 //= require govuk_publishing_components/dependencies
 //= require govuk_publishing_components/all_components
+
+//= require rails-ujs

app/controllers/applications_controller.rb

@@ -1,5 +1,5 @@
 class ApplicationsController < ApplicationController
-  before_action :find_application, only: %i[show edit update deploy stats]
+  before_action :find_application, only: %i[show edit update deploy stats destroy]
 
   include ActionView::Helpers::DateHelper
 
@@ -74,6 +74,11 @@ def update
     end
   end
 
+  def destroy
+    @application.destroy!
+    redirect_to applications_path, notice: "Successfully deleted application"
+  end
+
 private
 
   def find_application

app/views/applications/_form.html.erb

@@ -61,5 +61,17 @@
 
   <%= render "govuk_publishing_components/components/button", {
     text: @application.new_record? ? "Create Application" : "Update application",
+    margin_bottom: 4,
   } %>
 <% end %>
+
+<% unless current_page?(action: 'new') %>
+  <%= form_for @application, method: :delete do |f| %>
+    <%= render "govuk_publishing_components/components/button", {
+        text: "Delete application",
+        destructive: true,
+        data_attributes: { confirm: "Are you sure you wish to delete this application?" },
+        info_text: "You can remove this app if it is retired.",
+      } %>
+  <% end %>
+<% end %>

test/functional/applications_controller_test.rb

@@ -326,6 +326,24 @@ class ApplicationsControllerTest < ActionController::TestCase
     end
   end
 
+  context "POST destroy" do
+    setup do
+      stub_request(:get, Repo::REPO_JSON_URL).to_return(status: 200)
+      @app = FactoryBot.create(:application)
+    end
+
+    should "delete the application" do
+      assert_difference "Application.count", -1 do
+        post :destroy, params: { id: @app.id }
+      end
+    end
+
+    should "redirect to the index page" do
+      post :destroy, params: { id: @app.id }
+      assert_redirected_to applications_path
+    end
+  end
+
 private
 
   def random_sha