Add AdminNotification management at admin panel

In the same fashion Newsletters is managed, with the only difference that
the preview is using the notification partial in the same way the index
of notifications.

app/controllers/admin/admin_notifications_controller.rb

@@ -0,0 +1,67 @@
+class Admin::AdminNotificationsController < Admin::BaseController
+
+  def index
+    @admin_notifications = AdminNotification.all
+  end
+
+  def show
+    @admin_notification = AdminNotification.find(params[:id])
+  end
+
+  def new
+    @admin_notification = AdminNotification.new
+  end
+
+  def create
+    @admin_notification = AdminNotification.new(admin_notification_params)
+
+    if @admin_notification.save
+      notice = t("admin.admin_notifications.create_success")
+      redirect_to [:admin, @admin_notification], notice: notice
+    else
+      render :new
+    end
+  end
+
+  def edit
+    @admin_notification = AdminNotification.find(params[:id])
+  end
+
+  def update
+    @admin_notification = AdminNotification.find(params[:id])
+
+    if @admin_notification.update(admin_notification_params)
+      notice = t("admin.admin_notifications.update_success")
+      redirect_to [:admin, @admin_notification], notice: notice
+    else
+      render :edit
+    end
+  end
+
+  def destroy
+    @admin_notification = AdminNotification.find(params[:id])
+    @admin_notification.destroy
+
+    notice = t("admin.admin_notifications.delete_success")
+    redirect_to admin_admin_notifications_path, notice: notice
+  end
+
+  def deliver
+    @admin_notification = AdminNotification.find(params[:id])
+
+    if @admin_notification.valid?
+      @admin_notification.deliver
+      flash[:notice] = t("admin.admin_notifications.send_success")
+    else
+      flash[:error] = t("admin.segment_recipient.invalid_recipients_segment")
+    end
+
+    redirect_to [:admin, @admin_notification]
+  end
+
+  private
+
+    def admin_notification_params
+      params.require(:admin_notification).permit(:title, :body, :link, :segment_recipient)
+    end
+end